Whilst acceptable for dps or healers to turn up. A tank should actually know what they are doing. Tanks that turn up the moment they have ilvl, know very little or nothing about the boss, and/or generally bad, can and should expect some abuse. Its no fun queueing for a long time as dps, why should we tolerate it. I have never had a tank ask about tactics/advice, generally if they are bad they are immediately on the defensive. Knowing full well they can requeue swiftly.
I'm not saying that all tanks are bad, same can be said with healers and dps. Bad tanks will screw a raid over, healers and dps do not. If noob tanks come in with the attitude to learn and be humble, they deserve to be treated with respect. Problem is i have yet to meet a tank that is like this good, bad or ugly.