I will concede that tanks staying in tank stance for the duration of large pulls is a fair point, but you know why tanks should be expected to at least TRY to DPS at least for bosses? Because it's incredibly easy. There's simply no excuse. In your average roulette/24-man/8-man, there is no such thing as a boss that will execute you instantly for not having tank stance up. Neither will your healer experience a sudden, crushing, overwhelming burden of healing 20% more incoming damage. Even then, defensive cooldowns exist for the purpose of mitigating more than enough damage. So, why not press the button that takes off the damage penalty? Is it laziness or ignorance? In either case, both are undesirable, and by extension, make the tank undesirable.