I understand that very well.
1. It's a nightmare to balance for competitive if you can have doubles. And i'm not talkin' about lame 5x same stacks but 2-2-1 stacks of 3 heroes or 3-1-1 in various combinations. Just look at all the Mobas out there always trying to catch their outliers and keep ever champ/hero on equal footing.
2. It's better for the viewer and that's where the long term money is. They didn't manage to do it right with HotS and SC2s viewers are okay-ish but with OW they've now the chance to stand next to Dota, League and CS - so you obviously want viewer friendly games and havin' each player on a distinctive character makes for easier recognition on stream.
3. If they manage to get their kits to comparable strenghts it doesn't take away too much strategic depth - again i refer to Mobas where picks are incredibly important even without doubles and there's enough metagaming to be done that way.
