I haven't played in a while, but in my experience when that happened it either a) wasn't a problem because the group was geared enough and/or good enough that comp didn't even matter or b) the person who queued and failed to actually perform their role was asked to swap or kicked. I think I saw maybe one or two groups where it actually became a big problem? In dungeons anyway. Healers not healing in lfr was sure a thing, but I mean hey, it's a problem in FF too in my experience :P Had more problems because of different flavors of idiocy than that over the years that's for sure. In both games.
Anyway it'd be nice to be able to do this at least in premade groups. Had a full premade ex roulette the other day where we accidentally queued with two healerswasn't really a problem and we finished the run reasonably quickly (blood for the the blood lily!) but still would've been nice if one of em could've swapped without having to drop out and requeue.