Unless the factions are created so asymmetrically that skills from one faction won't necessarily translate to another(i.e. comparing terran and zerg), you honestly need to play more than one faction at a relatively high level to understand the inner workings of the faction. However, the ability to focus primarily on just one faction means that even though you can play the other factions at a high level, you'll be able to play your main faction at the highest level. Which, in turn, elevates the level of play overall.
For that reason, specialists are often not quite on level with a somewhat-less-of-a-specialist, at least from what I've experienced.
And forcing a player to master two armies won't elevate the level of player overall by giving him a deeper insight into not just one both both armies ? By allowing mirror matches you do create a much more narrow player. And also one who might be a bit more limited when it comes to providing feedback for balance since they will only consider one side of the argument.
Again, it's a design issue more than anything else. TvT is/was one of the deepest match-ups in SC2, providing each player with a massive variety of options and styles. Terran was, by far, the best designed faction of that game, though. Contrast that with protoss mirrors, and then consider how well protoss were designed compared to terran.
Obviously it's still going to be harder to design around mirror matches, but if that's something Relic has had in mind all along, it could still work out if they're wary of what they're doing. And according from what I've heard from people that tried the alpha, they didn't experience any problems with the mirrors contra asymmetrical matches. Thus, I'm optimistic, though still open to removing mirrors if they turn out to be crap/too much work.
So me pointing out that they were bloody boring doesn't count for anything ? And wouldn't it rather be the point to balance the game and design it around the asymmetrical gameplay. That is the more dynamic of the gameplay modes. And rather what fits in more with the first CoH.
And you do risk that things which do not work in mirror matches but work well in asymetrical matchups could result in things getting removed. Simply for the sake of the mirror match.