That doesn't actually make sense. Both of those are the same thing. The -ONLY- ways to control how long OR how many casts you get off while at high mana is to A) get more mana or B) lower your rotation and as such, your dps.
Regardless of how much of any stat you have except for intellect (and mana, duh), you will ALWAYS get the same number of casts off and be at the SAME percentage of mana. Spell cost doesn't change regardless of any stat. Haste makes you cast faster, which just brings you to lower mana faster. You don't get more casts off at a higher mana percentage from haste or any other stat except for intellect/mana.
My own personal thoughts on why haste is a worse stat than crit is simply this: Crit assists the benefit of mastery while haste and mastery do not grow off of eachother. If you crit more with 20 mastery, your 20 mastery gives you -more- damage than it would have if you had less crit. It works that way for all stats but haste. Now, haste comes out ahead in cooldowns and procs. You get more casts off with your DFO, phylactery, mana gem, AP, or any other proc out there and you gained a little extra dps from haste. The simple fact of the matter is crit -also- gains from that. If you crit more with a proc or AP up, you gain extra benefit from your crit.
Regardless of whether my reasoning is right or not, haste is simply less damage per point than crit and that's why we reforge it. It doesn't make haste bad, it just means it's worse than crit.