I played as a healer in many MMO and i have a lot of experience over this argument.
I also enjoy my Jedi Sage, she's adorable while saving the lives of many players. It is a bit hard to heal compared to other games, but nothing impossible, just needs practice.
As for "healer needs too much time to kill some one npc"... you should've thought of that when making a healer, they are allways like that, in every game.