Healing is the backbone of any group success, whether it's self-heals (like a healing-hands war) or totally on the backs of monks. Casters not being a able to kite damage or notice when they are surrounded by warriors, just doesn't bode well. If you have a ranger, tightly packed groups should always be pulled, if not the next best method is to wand aggro, sending a tank in with that sort of situation always tends to spell disaster.
I honestly don't think any one proffession is the best or worse (noob or pro) to have on a team. Typically if you're going with an organized group, you know what your healing status is before you get set, you know whats most affective and how the team will most likely work together. Many times i know things are going to go bad from the start when you have a war spamming GOGOGOGOGO. No monk can totally heal stupidity and a war that thinks he can tank everything with no stances or self heals whatsoever is a dead warrior, a caster that just HAS to get off metshower while 3 wars and a ranger pound them into the dirt, simply has it comming