Lets speak about Mentor's.

[BillyKaplan]

I don't want to be a bad example...there are enough of you out there.

You're being pretty horrendously rude and generalizing right there, so. There's that. Not justifying the language Starflake chose to use in the least, but come on.

So a player is not qualified to see if someone plays bad just because he didn't level the classes, very interesting

While some things are glaringly obvious, you'll often hear complaints about performance from people who don't know how other jobs even operate. One of the most prominent if basic examples is blaming RDMs for lack of heals/raising when they're lower than the relevant levels for those skills. So while I concede you can often tell how bad someone is without having played that specific job, or even their role, you'll also very often see people who complain without understanding objective restrictions holding performance back.