It's not always that easy. For example, if a group is ignoring basic boss mechanics and standing in things that hurt, so that they're overly taxing their healer, it's impossible to keep them all topped off all the time.
My sorc healer is well geared, and it's easy to keep the whole group topped off... unless things go bad with the tank or DPS, in which case one of them or more might have low health at the end of a boss fight. My priority is to keep everyone standing with the tank as the priority, not keeping everyone at 100% always. If it's a nasty fight but everyone's still alive at the end, I did my job. :P
Exactly. I can't control what the other members of the group do, or how intelligent they are (okay, I can control the tank, because he's my BF and we pretty much only run together now), so if one or more DPS stand in an AoE or get spit on, and at the same time the tank is taking unavoidable damage, the tank is going to be top priority. I will TRY to heal the DPS(es) once the tank is in safe range, but if they're still standing in AoE's (despite telling them not to after the first surprise) they might die before I can get off a heal for them. The tank is my top priority because if he dies I'm almost guaranteed to steal the aggro (and while I'm pretending to be a tank and healing myself, the DPS(es) will STILL be doing stupid things and acting like I should heal them).
This NEVER happens while running with experienced people who know not to stand in things. I usually keep everyone at near 100% health throughout the fight, which means we can run right along to the next mob without pausing. It's the lower level FPs that cause the problem, and I don't know if it's new players who don't know the mechanics or people running alts who forget they're not overgeared out the wazoo like they normally are, or a combination of both. But people do need to show some consideration to their healer and stop acting like healing between fights is part of the job description (if you want to take on that job for the challenge, far be it from me to stop you, but people need to be prepared to show courtesy to the other healers that don't (or can't in the case of commando medics who have limited power cells) want to).
Same goes for DPS pulling before the tank is ready. Stop it already!
And the same also goes for people who are stealthers in HMs and STILL haven't learned that boss mobs have a mechanic to allow them to see a person in stealth from further away than non-stealthed people (it keeps stealthers from skipping bosses). Have had more than one boss fight start before everyone was ready because someone stealthed up to the group and just stood there. By the time you get to an HM you need to know about these mechanics existing - and if you didn't before, you do now, so stop doing it!