On one hand for DPS queues it would be nice, on the other hand healers have to put in more work to heal that extra man. I'm not against the idea and personally would like it, but looking at healers perspective some might find it overwhelming to keep 6 or even 5 people alive.

I'd definitely be down for a "fill" option as I play multiple roles and it would alleviate some of the queue times when party only needs specific role.