Dispel

It comes in handy from time to time in the earlier games, especially against any enemies using Haste in VII or VIII. For the most part in VII, though, troublesome enemies use Barrier/MBarrier/Wall, which can be taken out with De-Barrier, the weakest spell on the Materia which includes De-Spell.

FFX... I don't recall using Dispel much, but it was probably handy from time to time, especially in Arena battles. But I don't really recall.

FFXII is where it truly became an 'unmissable' spell for various major battles. I'd never have beaten Yiazmat (the strongest optional superboss) without Dispel, since that boss is fond of giving itself Shell, Protect, Regen, Faith, Bravery and who-knows-what else. Lots of regular enemies, like the Archadian Judges and infantry, use Shell or Protect; this can make the battles quite risky when there are lots of foes and you've got a low-level party.

I found it to be a very useful spell in FF12, but other than a few battles in FF10, I don't really think I used it in any other FF game. It isn't useless or pointless, but it's just something that helps with certain battles.

I've found Dispel to be useful in X, although only very occasionally. For instance, when an enemy uses reflect on itself then, like the Seymour on Mt. Gagazet I think it is, attacks themselves to reflect it onto you (a pretty stupid strategy if you ask me). Also, when say, a dark element uses reflect on itself I like to use Dispel. Mostly because instead of beating them down physically I like to use Lulu as well to give everybody a chance in battle.

But for the most part Dispel's pretty useless.