Does the game become more competitive if balanced mirror matches are included, or just more diverse?
If we want e-sport to be a bigger part in COH2 than in COH1, then yes. Because mirror matches make the organization of tournaments and other competition a LOT easier.
At the moment you are somewhat forced to do a Best of 3 format. The first 2 games are necessary to define a parameter (VP lead) which then decides, who has the unfair advantage in game 3.
Not only is this unfair in itself, but best of 3 matches are long! This makes it a lot more time consuming not only to play such tournaments, but also to organize them, because any bigger event expands over more than one day.
Leaques are almost impossible, cause they require a lot of different players to find an overlapping timewindow to play their games.
With mirror matches, best-of-1 games become totally fair. This lets us get rid of an articifial parameter (VP lead), and allows us to have more competition.
To implement mirrors into tournaments, they already have to be part of automatch, which is where the metagame evolves and strategies are developed. It goes hand in thand.
Put authenticity into singleplayer/custom games, but let the competitive part use its full potential!
Now there will be some people saying: "There is only a small bunch playing in tournaments, blablabla. Mirrors ruin it for everyone else, blablabla". But, this is the wrong perspective.
Events like Sunday Night Fights have shown how much of a followership is connected to such competitive events! All these people are involved, too. And the more of these events, the more visible COH2 will be, so it will grow.
You probably don't know how many new players the SNF shows brought to COH1. They jumped over it on the twitch.tv main page and wanted to play it themself. They post in the chat "what is this game called, it looks awesome". Additionally to advertising, this is how games get big!