Strongly disagree with this.
You should be able to play every single offense, although you can get away with knowing either tracer or genji.
There are too many counters in this game. You are a worse player if you cannot deal with a strong player on the enemy team.
pharah, for example, is unstoppable if no one has the range to deal with her. Suppose you have a lucio, reinhardt, reaper, symmetra, and junkrat on your team. but you only main tracer and reaper yourself?
Your entire team will be annihilated by pharah because no one has the range to deal with her. In a game where you can expect your teammates to not counter enemy players...you should really really know how to play soldier here. (He destroys her) (By the way, roadhog and d.va can deal with pharah but you might be expected to DPS and cant pick these characters for a proper composition.)
With tanks, you should know 1. Reinhardt for payload pushing and torb + bastion protecting. he also works well on a team more focused on front lines (Soldier + mccree) because these characters can shoot through his shield.
With support. You need to them all. However, zenyatta is slightly undertuned and you can get away with not knowing him right now. But i expect buffs.
Reasons: 1. You should know lucio. Lucio deals with flankers (Reaper, genji, tracer..and to an extent mccree) much better than other healers. Lucio is also really fantastic on maps where he can push people off (looking at you lijiang tower) 2. You should know Mercy. Mercy is very potent with an allied pharah or sniper. She is very important on anubis on point B defense because she an hide behind the pillars (behind point B) and revive her entire team. Healers are really dependent on maps. For defense You want to know a sniper (either hanzo or widowmaker) you want to know at least one builder you probably want to know how to play symmetra
It'll take a long time to learn all these characters, but as someone who has played hundreds of hours in the beta, i firmly believe my ability to adapt by changing characters is more important than my mechanical skill in-game.