It isn't. Haste is usually the very last modifyer applied to a cast, sadly . It would scale awesome for SF else, and even better but capped quite fast for Wrath because both get .5s off by talents. If haste was applied on base casttime, it would very likely outclass almost any other stat by quite a magnitude.
Haste >>>> Crit
Depending on your mana regen.
Since people with low mana regen better not get alot of haste, since they will just run oom really fast. More haste = more casts = more chance on crits = more dps. But if you have a low mana regen and you're planning to not get any spirit in, get crit gear and enjoy the mana regen from that. (Don't spam SF then -.-)
I'd rather say it's the other way round, but non is the case. Haste does say it reduces casttime by a certain percentage, and it still does with NG to the same extent. The way it works is actually the most straight forward one, because this way you can really say: 1% haste = 1% DPS (for Wrath until GCD-capped ofc, and not factoring in lag in general).
It does however not exactly match this 1%DPS if buffs of limited runtime (unaffected by haste) come into play, like Eclipse, as soon as your haste would enable you casting one spell more within the buffs uptime. But this really is in the realm of nit picking