That's the thing, if you compare it to the other simic level 1 abilities, the intention is that they each be occasionally situationally useful.
However, unlike the other 1st level simic abilities it's quite easy to build a character who the answer to "how often is it useful?" is "most of the time". All you need is a class ability that allows you to propel yourself vertically upwards. And there are many of those: Fly spell, druidic shape changing (after level 8, change into a bird, fly up, revert to natural form, glide), Spider Climb spell, Fey Step, Storm Sorcerer ability, Misty Escape (Warlock), Tiger Totem, Eagle Totem barbarian abilities, and I'm sure there are more I haven't thought of.
As for "why" you would want to use it, the important thing is not the maximum speed, but the fact that it is "free" movement, in addition to what you can normally get from your move and dash actions. So you could use it to:
1) move between melee targets that would otherwise be too far apart
2) close rapidly on an archer
3) Stay well out of melee range, even against superfast enemies
4) surprise a target by beginning a round well beyond their visible range
Okay, let us first break some of this down.
Fly - 3rd level spell, means it comes online at 5th level and costs a daily resource
Druid - Comes on 8th level and costs a limited short rest resource
Spider Climb - 2nd level spell, comes at 3rd level and costs a daily resource. Additionally, requires a wall in the correct positions
Fey Step - Eladrin Racial ability... if you know of a way to be two races at once let me know. Otherwise we are talking about Spring's ability to use a 1 per rest ability to move an ally, which requires two players and two races.
Storm Sorcerer - Bonus action when you cast spells, a legit way to activate for limited resources.
Misty Escape - 1 per rest, comes online at 6th level, and requires you to be hit first
Eagle Totem - Comes online at level 14
Tiger Totem - I guess you are talking about gaining an extra 3 ft from jumping up while raging? At the opportunity cost of other 3rd level totems?
So, most of these are mid-level and cost some significant resources, but more interestingly is your claim of "free" movement, because generally none of this is "free". Take using fly for example. An action to cast, 60ft of movement up, and then you cancel concentration, burning the spell, and all you have gained is an additional 60 ft of movement laterally. This movement is only a bonus in the fact that it would have taken you an extra turn to get that far if you had simply flew in the direction you wanted. So, we are comparing (in this instance) a turn of movement vs the versatility of being able to move in 3-dimensions for the length of the spell. That extra round is very rarely game-changing and that is all you are gaining for burning a 3rd level spell slot and the benefits of flying (such as staying in the air out of the reach of most opponents)
Additionally, before I address your 4 scenarios, we must remember an important point about the movement of the manta glide. It is 2 ft forward for every single foot down, but you still must move through the space. So, to fly over the heads of most humanoids you would need to cover their 5 ft square (minimum of 2.5 ft drop) while also being at least 5 ft above the ground. So, if you were to teleport 30 ft up, you would only move 60 ft across if there were no medium sized enemies 50 ft away, because your last bit of movement would be canceled by running into them. Additionally, falling those last 5 ft (minimum clearing distance) moves you 10 ft, out of their range and triggering attacks of opportunity.
With that in mind
1) If you have targets that far apart the battle is either nearly over, or so large to be unwieldy. Also, most of the classes which can activate this movement are spellcasters, and few spellcasters want to be in melee to begin with nor do they have multiple attacks per round. Most Likely candidate is the Eagle Barbarian, who needs a target 45 ft away and they need to have no access to throwing weapons (since nothing prevents a barbarian from attacking in melee, moving, drawing a throwing axe, and throwing it). Also, it is likely largely irrelevant if the character can get to the second target, as they have an entire team of people.
2) Can largely be done anyways, the distance covered is only gaining you a single turn. The archer may attempt to kite, but if you couldn't close on them without this, then you are likely not going to get more than a single AO on them anyways, since the archer will just book it instead of sticking around for melee. Many of your solutions are only usable once remember, and again, this is an option important for melee characters, most spellcasters you can reliably cause this to activate have range themselves, and generally enough range to hit archers. This also runs into the problem of terrain or other enemies which may impede your progress due to their height.
3) Being able to fly or climb up generally does this as well, in fact getting out of reach would be far more effective since most of the time you could be caught up to anyways. Casting fly and then canceling it to fall doesn't help you on round 2 or 3 when the enemy has dashed to close the gap. Also, I'm curious about teamwork here again, if you are moving so fast away and the enemy is incredibly fast and chasing you.... your allies are getting farther away and will be unable to help you in the situation. You are essentially choosing to split the party and solo something that you are trying to flee from.
4) Requires limited sight, which means darkness, which means you have limited sight as well. And, again, this is only a good plan if you solo surprising the enemy is a good idea. Also, most of the powers you listed are kind of worthless for this in addition to it being a solo fest for round or two. For example, a Druid shapechanging into a bird then using their bonus action to transform back and falling into the enemy ranks. You could have just as easily turned into a small bird, flown into the enemy ranks, and transformed back, gaining you even more surprise since a giant man-sized figure slowly falling on top of you is less surprising than a sparrow becoming an enemy.
And, I want to remind you, this started with talk about this ability being "way too powerful". Yet, none of these examples are really situations that come up a lot, and when they do they are not often horrible situations the players must survive. Also, using this ability either requires many environmental factors to line up or the use of other abilities which are already useful in their normal context and will rarely be more useful with this addition, because DMs will rarely set up scenarios where that extra amount of movement is needed.
Why would anyone choose one of the other two highly situational abilities when you can choose something that is useful pretty much all the time?
Seems like a pretty unintentional consequence to me!
(even without special class abilities, you could end your movement with a running high jump to gain an extra 3+strength mod movement each turn.)
Because being able to climb most surfaces, making ranged characters outside of the reach of melee types, is incredibly useful in a lot of situations. Also, for sneaking into locations, climbing can be incredibly useful.
Because being able to breath underwater can be incredibly useful, especially if your DM indicates there will be plenty of chances to infiltrate while underwater or something similiar. Also, stealthing underwater for areas such as docks and sewers can add huge bonuses to attempts on getting somewhere unseen.
You are talking minor gains in distance on entirely flat surfaces with no enemies in the way, versus the ability to go where you normally cannot go, opening 3D movement or other areas of the map.
Indeed, in the Races of Ravnica Dragon+ episode posted earlier in this thread, Crawford goes a bit into how the vast majority of players make entirely narrative driven decisions with no care for mechanics or optimal results.
Also, it really isn't optimal in a lot of circumstances. It's a really minor benefit most of the time from the things I've seen in game.
