In this particular instance, the low level wizard was highly useful.
I'm really glad that this worked for your game. Seriously.
Let me tell you about my game yesterday. The PCs found a magic rapier and two rooms later they found a fabulous treasure room. They have 3 days left of food remaining in Undermountain. 8000 GP, 25,000 SP, 6 magic swords, 500 gems, etc. Now, this is supposed to be the treasure room of this famous NPC and the room before it was his crypt. The party is seriously depleted on daily resources and they decide that this is just too good to be true (now granted, they used metagame knowledge here: "There's no way KD would give us this much stuff at level 5"). Something must be wrong with this, so they rest up, get spells back, and once prepared, the wizard casts Identify on the swords and rapier and find out that the rapier is +1, but the rest is fake. They use Investigation (which I allow them to use as 5E appraisal), roll well, and determine that the coins seem to be the wrong weight and that the gems feel like glass. They determine "Fake treasure room, fake crypt leading to treasure room".
3 PCs out of 7 use a rapier, so they decide to give it to the Paladin. The Paladin attunes to the rapier and I give a description of oily black smoke entering the Paladin and the Paladin feeling a presence in her mind. They now have 2 days left of food. They are trapped in a crypt with traps and guardians and soon, their only food supply will be a third level Cleric spell. If the Cleric dies and they do not find a way out, they will eventually die of starvation. The Paladin player wants them to hole up and try to get rid of this item (she correctly thinks it is cursed), but the party is feeling a time crunch here and decide to continue on without acquiring new spells which might get rid of the item.
In a fight later that day, the Rogue puts up a Moonbeam spell from an item and the Paladin ends up taking damage from it. The rapier is a Rapier of Vengeance (and ironically, the Paladin is a Paladin of Vengeance). So, the Paladin fails the Wisdom save and goes off to fight the Rogue. Now, the Paladin is a tank and very hard to hit with attack rolls. She has +1 Plate, with the Defense fighting style, shield, and the Defensive Duelist feat. She has AC 22, AC 25 if parrying; and with Shield of Faith up, AC 24, AC 27 parrying. The Int 8 Fighter calmly walks over, grapples the Paladin (success) with one attack and knocks prone the Paladin (success) with another (his odds of each was about 50%, so 25% to manage both). The Rogue then tries to subdue the Paladin (advantage due to prone), criticals, and knocks the Paladin unconscious with 30+ points of damage (Paladin was already a little bit hurt). The Paladin rarely gets hit, has never gone down, and these two took her down in a single round (pretty much surprised the entire table, granted, luck played a huge factor here).
My point on this. The game designers of 4E put a bunch of Forced Movement powers into the game because they felt that having one or two hard to hit guys holding a front line choke point while a bunch of PCs/NPCs on either bunched up and used potentially weaker ranged attacks was a boring way to play. They wanted both sides to move foes around in order to allow for new tactics and new possibilities and not just have the game played like Checkers. 5E does not have all of the forced movement features of 4E, but DMs should have NPCs use what the edition allows.
Holding a choke point in a lot of encounters can be effective, but it leads to long drawn out boring fights. At least IMO. Fights should be surprising and interesting and full of the unexpected with foes from both sides getting attacks nearly every round. I don't play D&D to have the same old tactics be used over and over again, but to have new stuff happen. The DM can control some of this with terrain, environment, NPC selection and NPC decision making.
But if the DM has the NPCs do two types of actions in the game: "Attack and Dash" or "Disengage and Dash", then he's missing out on a key element of adding coolness to encounters: "NPC decision making".
Our Int 8 Fighter helped drop the Paladin. Not by just swinging his sword, but by wrestling her to the ground. NPCs should do these and similar actions too. They have experience fighting other creatures and should not just be cookie cutter bags of hit points.
Simple example (similar to Celtavian's example, but not necessarily the same), 1 = PC 1, 2 = PC 2, S = flaming sphere, abcdefg are enemies
Code:
_________
| S c e g
|ab__d_f___
|12|
a and b are getting roasted by a flaming sphere and cannot leave without provoking. What do they do?
Grapple / pull
Like the more powerful gnolls in Celtavian's example, these foes have Str 16, but no athletics skill, so +3. The PCs are level 3, so presumably have +5 in acrobatics or athletics (or it could be +3).
Code:
_________
| S cbe g
| _2d_f__
|1a|
The odds of this being successful on a single attack action is 38.25% (47.5% if both PC and NPC have the same modifier to the roll) and only PC 1 gets an OA. PC 2 now has 3 immediate attackers on him and if he does not escape the grapple, can be pulled even further down the corridor on the next round and totally surrounded. The flaming sphere could be used to attack foes, but that brings it withing range of PC 2 and foe "a" now prevents the line from being reformed.
Alternatively, if b misses a grapple, a could do a shove and push 1 or 2 away, and in either case, c through g could now congo line through the opening (with a few potential OAs).
In the case of a and b being gnoll pack leaders, someone might say "Why would Int 8 gnolls do this?". My first answer is: because the DM is not playing checkers, and the two gnolls in front are also being fried by a flaming sphere, so why not do this?. Thematically, my answer would be "Gnolls are dog-like pack creatures. Their first instinct is to pull a weak member from the herd, surround it, and rip it to pieces.". This is exactly how Gnolls would fight, for example, against those pesky Dwarven lines.
Why play your NPC monsters as: swing, swing, swing? Boring. Use Grapple. Use Shove. Use Dodge. Use every single action type in the PHB, not just normal attack, disengage, and move. Monsters/NPCs should use all of the options in the PHB, just like PCs. And this does not even include optional rules from the DMG. To me, the DMG is there to help the DM make encounters even more memorable. If something there looks like it works (like Overrun), use it.
And don't let your players always dictate where and when encounters are going to occur. They only get to make decisions for the PCs, not the NPCs. This does not mean that the NPCs always make the best tactical or strategic decisions, but it does mean that the PCs do not get to make those decisions for the NPCs (or complain if an NPC makes an unusual decision). The PCs fake out the foes by letting one escape and hence drawing the rest into a trap. Sometimes that works, sometimes it doesn't. The first casualty of any battle is often the plan.