See, here's what I don't get. Sure, the party took him for granted. It wasn't that he couldn't heal, but that they were so incompetent that he wasted all his mana healing them, while also being a second vanguard and DPS. (The second healer pretty much confirms this.)
But why did the Healer's School kick him out for being incompetent? According to the second healer, being able to do things that the party claimed the MC did should be impossible. So, I don't understand why so many people seem to think that he is useless, unless I just chalk it up to bad writing (which is probably the most likely answer).