Well if that's all, why bother?
Heh, I said simply, not only. I checked out the old version and you could cast it on other people, but it was +4 enhancement to Dex, base Reflex save bonus of a rogue equal to your character level, and Spring Attack feat. So a flat +10 to Dex is more simple. This is like a 5th level spell with 1 round/level duration, so its probably not too bad.
Armor max Dex bonus only applies to dex bonus on AC, you still get the bonus for reflex saves, skills, ranged attack rolls (or melee attacks with weapon finesse), etc. So you would still want it, although obviously it is more benefit with a higher Dex bonus. Celestial chain has a +8 max Dex bonus, thistledown padded has a +10. ELH has armor of the celestial battalion with a +10 max Dex bonus, and magic of faerun has the Nimble quality you can add to magic armor that ups the max Dex by 2 (which would stack with mithril or thistledown armor). Then of course you might just use bracers of armor, Dyrr's impervious vestments, or mage armor/greater mage armor so you would have no max Dex bonus being unarmored, and/or a monk (either a multiclass monk like sacred fist or maybe your cleric buddy just casts it on you b/c it is after all a touch spell).
Anyway, who cares. Now I forgot what I was thinking about.
Btw, I think the reason for a +10 to Dex being a big deal is you can't really get that high w/o this spell unless you were an epic char. Like say gloves of dexterity +10 would cost 1 million gold. If you make a lvl 20 char, you get like 760,000 gp, and the most expensive item you can have is only 1/2 that amount. Also anything over a +6 to a stat is considered epic. There is the Strength of my Enemy power that can give you a +8 (or higher with augment) to Strength, but that is only after attacking the same person with eight succesful melee attacks.
However, there are some other polymorph type spells in this book that temporary turn you into a lycanthrope, ie wererat or werewolf, which have huge enhancement bonuses to stats, so they might be going away from the idea of over +6 is epic, hmmm... but after seeing the effects of really high stats in a few of our games, I'm not so sure that would be a good idea.