I watched a moment ago again a Twitch video with an english Bless online player (JP). The Tank was offline (DC or dunno was offline). There were 5 adds to Healer for heal aggro then. And he didn´t could rezz the dead Ranger, cause of adds. And the Player couldn´t wait or had any appreciation for this situation.
"Rezz me b**ch" , "Heal me!" or "Your fault" are standard patters for any inanely DPS/DD and sometimes Tanks, too.
A Paladin is more specialised to focus on one person to heal and to shield the party and only has a fistful of AOE-healing.
A Mystic is more specialised to focus on AOE-heals, but not as strong to heal permanently full life to everyone.
But both healer classes CAN´T heal every s*it in any situation. Nor PvE, nor PvP, nor RvR.
It is not only in BO, even in other games random DPS/DDs/Tanks gripe often about the Healer.
Yes, there can be bad people, but lots of them try their best everyday. And to heal is not to say, that you only stand there all the time and press 1 2 1 2 1 1 5 like "3 button player".
A good healer is walking a lot of the time during a boss fight and has to enable wise any support/heal skill, even they have cooldown, yes my dear DPS/Tanks.
Every Healer class in most of any MMORPG has its limits to heal/support their party.
Why can´t they sympathise with it?
As a Healer you also have to watch whole the time to the boss, what it´s doing, where it´s going to, who has aggro and what is it going to do (mechanics etc.).
Supplemental a Healer has to look out for party´s lifepoints and negative effects to party cleanse, own and party´s MP and in some games even to dispel the boss positive effect, plus to keep up buffs/shields.