I think some are missing the mark a little bit when they say it's about responsibility, it's not responsibility in my experience, it's people don't want grief. If you make a mistake on a healer or tank it's much more visible than on a DPS, usually. I know people who have responsible jobs, mortgages and kids, they still don't want to tank/heal. It's not responsibility that they don't want to deal with, it's getting chewed out by players with bad attitudes because everyone thinks they know what the tank/healer should be doing, despite not being prepared top play it themselves. They just don't want to deal with that after dealing with actual responsibilities all day so it's easier to wait in a slightly longer queue and just pew pew while nobody takes any notice.

In truth I've had very little aggravation while playing tanks but players who aren't used to tank/healer roles see it as too easy to get flamed.

That's my 2c, for what it's worth.