This, an simple need or greed function like in wow would solve most, yes if some asshat always need everybody else needs on the later ones.
WOW had plenty of balancing features, I remember one episode well, I was shaman and had signed up for dps or heal in random dungeon while questing as dps.
Pops in halfway trough an dungeon as healer, Says: hi wait: need to change to healer setup, however the group was busy getting wiped.
Yes accidents happens, however because they was so mad I assumed they had kicked the previous healer for no good reason and was on an kick cooldown so I could enjoy myself.
Loot any trash mob, enjoy the scenery and comment on it then somebody did the gogogo.
Yes the downside was that you needed some intelligence to understand how the system worked, kicking somebody for doing 20% less dps or being gnome and you had some chance of getting something real bad as replacement.