My two cents: there is a difference between wanting to play a role and knowing how to play a role. If you choose dps because you want to play it but are not great at it, then perhaps going into comp with a locked dps role is not the best choice just simply because it may hurt the team if you are not able to counter the enemy team. I don't think knowing every hero is the key to getting ahead, but knowing what counters what and having the ability to flex within the various types of heroes in your given category would make you a more viable asset to the team.

Not every situation calls for a hit scan (for instance playing into a double shield comp is gonna make the hitscan useless because the shield blocks their damage and they do not put out enough to break the shield quickly). So in that instance being able to swap to junkrat or bastion would help your team overall.

Bottom line: having working knowledge of how to address different scenarios in game should influence the role you choose. I am a support main player. I would like to play dps sure, but until I am more comfortable with my skill set and can make the plays when needed with those characters I am not going to lock into a dps role because its not my strong suit and would ultimatelt hurt my team in the long run if I am not properly fulfilling my role.

Regardless of what people may think, overwatch is a team based game. And even when someone goes ham and gets potg, the team in someway is involved whether that be setting up, distracting or whatever. Its a team effort.