This thing is a bit old news now, I apologize, but recently there has been a lot of theorycrafting about using spell haste sets in arena, and now with S4 gear, you'll be able to rock 400+ spell haste easy without PvE anything. (There are also PvP honor pieces)
You can basically rock passive heroism all the time, or 30%-ish passive reduction, always. It costs you a good chunk of your damage. As fire, this isn't too attractive. Yeah, fast scorches are kind of cool, you get the debuff up faster, but really AP-Fire is almost entirely about using AP's multiplier to take advantage of stacked +dmg; the damage sacrifice is too severe.
What about sick DB? Well, you'd need 50% haste to be able to get a guaranteed Dragon's Breath into Fireball combo. But 30% haste yields only a half second window, which might be good enough. (Remember haste reduces not only Fireball cast time but the global on DB)
Either way, DB still sucks. =p
I see haste really shining in RMP WE 3s. With stacked haste gear and IV, it's not going to be feasible to deny your poly's perfectly. Plus you're going to freecast more bolts. Shatter combos will be much weaker, but WE isn't particularly dependent on your frostbolt damage to secure kills.
I think haste is a pretty cool stat in theory -- I don't think the class is set up very well to take advantage of it necessarily, but it's going to raise the skill cap a bit as healers (priests omg) are going to have faster globals and faster heals. When cyclones, fears, polys, flashes are all on the order of 1 second instead of 1.5, it's going to become imperative players learn to CS early in casts. (Your bars will also be unreliable!) Players will have more globals to use per game and more decisions to make.
The counter to all this is that mages without high damage can't kill anything or put much pressure on healers. Turning your frostbolts into Arcane Missiles might not be ideal. But think of the Focus Meta Gem + Quag'd Eye + IV + 30% passive haste + Heroism retardedness. Or just stacked poly casts in 3s!
Dicks.
What about 40/0/21 stacked Arcane Missile spam? =p
Also...
In a world with faster globals, recognize that all crowd control is indirectly buffed. Time isn't so much what matters, but globals. Imp CS becomes 3.25 globals instead of 2.5. Poly is 10 globals, not 8. Same is true for fear, scatter, trap, stuns, everything. Could be unpredictable how this impacts things; you'd expect faster globals to help warlocks, but since rogues lock out casters most effectively, perhaps they take as great a benefit in some weird sense, which increases rogue prevalence, which counter locks, which ate the spider who swallowed the fly and so on.
FUCK YOU I'M GOING TO GO SEE IRONMAN
