But then people would choose rock to beat scissors, etc. it's the textbook definition of imperfect balance.
I can maybe put this a better way. Rock Paper Scissors situation.
Scissors beats Paper. Scissors ties against Scissors. Scissors loses to Rock
In this situation, your choice benefits you in 2 out of 3 situations. You'll always beat paper, and the more skilled of the two Scissors users will have an edge in their encounter and win. Add on the fact that Rock's tradeoffs for choosing it are a bit worse than Scissors, and everyone chooses Scissors because a) it's slightly more optimized b) you aren't at a deficit for choosing it against other Scissors anyway.
edit: It's the same reason I have concern for the BR, DMR, and Carbine situation. Their functions will either be extremely similar and pretty much function as chooseable skins for your mid rifle.. or if they actually differentiate, people will just choose the most optimal one, and money/time/RAM has been wasted modeling two guns that nobody uses.