EDIT: Also it actually might be more boring having only 1 matchup in the game, USSR vs Ostheer. We'll be seeing the same matchup over and over again in tournaments because of that. This is what happened in SNF S3, we all know that US vs Wehr is the best matchup but we enjoyed some variety in game play thanks to the OF factions, and tournaments would be less interesting without mirror matches imo.
COH managed to stay interesting for a good while before OF was released, and even since then the preferred matchup for most tournaments still seems to be the vanilla USA vs Wehrmacht.
On the subject of the 'realism' argument: I don't think anyone can successfully argue that COH is realistic and that mirror matches would change that. It's certainly not realistic to begin with, for the reasons 12azor and others have detailed. The problem I anticipate is that in a historically-based game, a mirror match would pierce one's suspension of disbelief. Elements of the game very often deviate from historical accuracy and authenticity, but we manage to suspend disbelief all the same - it would be much more difficult to do so if the Ostheer is fighting the Ostheer.
I suppose that part of it comes from the fact that we subconsciously judge what is and is not a necessary concession to make a playable and enjoyable game; the success of COH has shown that mirror matches are not one of these necessities. The only difference in this thread is that we are attempting to consciously, instead of subconsciously, attempting to articulate this.