Now back to mirror match and people not locked in the box - a thought to consider:
Mirror matches gives a great tool to balance out and develop the metagame.
Players, in the process of learning and competing, sooner or later WILL find imbalances and easy/cheesy tricks/strats/builds. This fact was proven on so many levels and points in time of the CoH metagame development, that I think its out of discussion.
Now without mirror matches, these players are pretty safe, focusing on the single (most) imbalanced faction, staying there on the giving end.
Mirror match will not allow that, as they WILL ba matched against the very same faction, with the very same cheese. Time to try the recivieng end, fellas, no more safe spots.
Actually the opposite is true. Without mirror matches, those players would be forced to learn both factions in order to compete (by far the best argument against mirror matches tbh), while with mirror matches they can compete only having learned a single faction.
Also, the same 'cheese' strategies that work in a mixed matchup do not necessarily work in a mirror matchup, in fact I'd say it's very unlikely they'll be getting a taste of what they're doing to the other faction.
An additional thought to consider that I've yet to see anyone bring up would be expansions. While we can't assume anything, it feels very possible that in the future we will see expansions that introduce additional factions into the mix, just like with OF. With mirrors enabled, I don't see that ever being remotely balanced.