I think the benefit of Haste/Haste2 was reason enough to justify permanently dedicating a character's second ability slot to Time Magic. Both for the reason of hasting everyone of course, and for fixing Slow and Stop statuses. I don't think anything other than White Magic of course was so critical!
Is there an ability without which you wouldn't fathom beating the game? I think Haste is quite easily one of the best abilities in every Final Fantasy game it's found in, really!