I think what OP is saying, is that players main a class other than tank but tank for insta queue and the bonus which isn't much. I use to main both tank and heals but found tanking in this game to not be very enjoyable. I set a moderate pace but had some dps that liked to pull for me while marking with a macro and it got old. I liked healing so I went that way. I find I am much more tolerant of wipes, even the dumb ones as a healer than as a tank and can stick with learning content longer that way.