You may not have it yet (can't remember what level you get it at) but don't forget Rouse, is nice before the eos regen (whispering dawn?)

I wouldn't say it's absolutely vital right now, but the nearer to 50 you get the higher the need for Swiftcast, at 50 people will expect you to have it. It's not set in stone, but as general rule, it's usually better for you to be picking up the rezzes rather than the whm, due to it being quite mp intensive.

Up until the very last pre-50 dungeons, you can stay in cleric almost the whole time and be a third dps, with the fairy keeping everyone up, only hopping out of CS to throw out a few heals if things start going badly.