This is a question that may get better answers in the
Noob Questions forum, because most players actually play several classes anyway. For example, I have played both a coercer and defiler (among many other classes) for a long time.
Coercers are a useful class, whose role on raids is to provide buffs, power regen, and do dps mainly for a tank-focused group. (While coercers are an enchanter and thus can mez, there's almost no raiding content in which mez is ever used. There are also control effects like stun and etc., but most raid mobs are immune or have an extremely limited duration of 1-2 seconds; thus the roles listed above are the main ones).
Most raids only want 1 coercer, but some guilds can be flexible and use 2. (I'd say it also depends how hardcore raiding you want to do). Most raids can use multiple shaman, although mystics are a bit more well rounded with their buffs. Both classes are fun but coercer is alot less responsiblity. Healers in general require much more care because you have to watch other people's health, sometimes across the entire raid, plus their detrimental buffs, on top of curses, which only priests can cure. Coercers do have to watch power, but they mostly regen passively so while you're playing you're mostly dps'ing without much other responsiblity than getting the buffs on the right people when they die.