It depends on team composition, it's actually best to pick last to practice "filling" when you're in quick play to train for competitive. For example, maybe you enjoy playing the ass eater, but if someone has already picked the ass eater and you have to play the vanilla role because it better suits the team and balances it out, this will also give you practice with that role if you need to switch in the future depending on the next partner you encounters strength and weaknesses, you can't have a full team of ass eaters because if your partners team counter picks you to hell, then you won't have much of a chance to eat any ass because you'll be constantly sent back to the spawn point. Also you have to practice communication, it sounds like a simple thing but you'd be amazed at how many people lack this, for example, if you're all trying to eat ass at the payload but your partner sneaks past the choke point, then they're just gonna backdoor you when you could have easily just said "Okay, maybe we shouldn't be eating ass right now." when it wasn't the right time for ass eating.
tldr; You've gotta become a jack of all trades depending on the scenario at hand. If you're stubborn and continue to try and eat ass when your opponent is made to defend against ass eating, you'll be consumed by the dragon itself.
tl;dr; tldr; RYUU GA WAGA TEKI WO KURAU!!!!
Good question Nuke.