Never been kicked... but I have also never seen anyone be kicked.

I have seen people on 3 separate occasions leave because they knew they could not complete the fight (either connection issue or they just were not up to scratch and recognized they were pulling the whole group down), so they gracefully left and the DF provided us with a new DD.

Having a group mentality makes you a better player rather then the 'that is/isn't my job' mentality imho ^_^