1: BUT THE NUMBERS PROVE OTHERWISE!
If you have a 50% chance to hit the target
...then you probably shouldn't be blowing an Encounter/Daily on it. Cantrip away, you're not wasting anything.
Remember, the context I pointed out included Advantage. If your party can grant you some bonuses to hit and improves your odds of landing any given attack,
Witch Bolt becomes a much more appealing option, because you only have to hit with it once. If you don't have any increased chance to hit, you probably shouldn't be blowing anything with a significant cost on that gamble anyway. A single-target attack with a 50% chance to hit that costs a precious spell slot is just hurling resources down the pit.
Also, this % chance is a pretty flawed assumption. Bounded Accuracy means that in practice, there is a
rather huge variation in the actual chances to hit any given target, even with an on-par fight.
2: IT IS SO EASY TO COUNTER!
Chocolategravy said:
the spell ends if it runs past an object or steps out of range during part of it's move. Horrible spell.
Remember, the context I pointed out included being in melee with the front line. Not exactly the best place for being able to move around. You could swap in any immobilizing or movement-limiting effect for that. And, again, WB is more effective than many other things you could do in that situation, thanks to it being single-target (ie, it doesn't hit friendlies) and only requiring one hit to do multiple turns of damage without a save.
3: IT TAKES TOO LONG!
Chocolategravy said:
Anything that has enough HP to last enough rounds that it does reasonable damage sucks up all the encounter XP and so will be the only thing for the party to beat on... so it won't last enough rounds for the spell to do reasonable damage.
Remember, the context I pointed out was a "boss"-class monster. That is, a monster designed to be able to take a party's beating and keep on ticking. 1d12 damage is a whole lot more than the ZILCH damage you'd get on a miss.
Verdict
Overall, WB isn't great in a vacuum. But gameplay doesn't actually happen in a vacuum. If you can't be bothered to account for the context of an entire party being up against many different kinds of combats and how that might affect the desirability of the spell, that doesn't mean it's actually terrible. It just means your field of view is too narrow. Not everything is a wizard and a goblin in a blank 10 by 10 room. Most things aren't, actually.
I mean, I didn't think
unseen servant was any real great shakes, but I've got a DM who uses a lot of traps in his most recent dungeon, and this bad boy doesn't cost anything to appearify, so an extra pair of hands whose life doesn't matter has been worth more than my minotaur bard's weight in clerics. I've taken both Blade Ward and True Strike as well (though I haven't had much cause to use the latter yet), and I don't regret either choice. Which is just a roundabout way to say that I'm really suspicious of theorycraft claims of something's awfulness. Way too much variation and way too early to make an absolute statement like that, in my mind.
Ultimately, most of these Orcish Grandmothers probably have hearts of...well...if not gold, perhaps brass, anyway.