Mm, I am inclined to say scholars benefit more from crit and determination than spellspeed, although I do see an argument to build spellspeed as secondary since the pet buff increases your sspd by 20% if it crits. It still requires you to have the necessary crit gear for your summon though. Similarly, SMN benefit more from crit than spellspeed. I am not as familiar with the benefit to BLM, though it seems Mhaeric covered it quite extensively.

As for WHM and crit, while you do get bigger heals with crit gear, it becomes much more difficult to manage enmity with crit gear than spellspeed and determination. Most WHM already have a tendency to pull off tanks, and full crit gear will almost guarantee enough overheals to warrant a premature shroud of saints for most players. If you have ways to manage the extra enmity (largely how comfortable you are with the boss skill rotation and boomeranging tank hp) it's fine, but most find it too troublesome and focus on slightly more powerful and faster heals than the random multiplier of crit.