As a WHM, I'll heal anyone, but there DEFINITELY is a priority in my mind, and one bad judgment call can mean the difference between life and death.

Tank > Other WHM > essential members (with utility abilities or buffs that we ABSOLUTELY NEED to have) > others.

I will heal a BLM and melee DD equally... I'm not prejudice in that regard, but if I see a BLM going down fast at the same time as a melee DD, I may be more likely to make the judgment call to heal the BLM first because I know BLM are squishy and can't take a hit like a melee DD can.

But... if you are bad at avoiding things you NEED to avoid (like cracks, dash, plume, moogle aoes, garuda pushbacks, etc) and I notice it, all bets are off. Now, I'm not going to judge someone if they get hit once or twice. Everyone messes up or lags once in a while... but if you are consistently hit over and over again, it just doesn't make sense to put as much effort into keeping you alive as other people. I won't sit there and watch you die (especially if I am just standing there with nothing else that I need to urgently do), but being bad at avoiding these things AUTOMATICALLY drops you to the lowest place on my 'priority' list. I'll still keep you healed, but you are my absolute last priority at that point. I hope this makes sense.

This isn't a 'system' for me. If a person is new, or they apologize and understand they are getting hit, or say they are lagging, or explain that they are trying this fight on that job for the first time... I will be a bit nicer about it. BUT, if this person has a bad attitude, or is downright denying that they are the problem, I no longer empathize.