Hell i could point at the vast sea of mods which add in a lot of things. but never mirror matches.
That wasn't because people didn't want to add mirror matches in their mods, it was because it wasn't possible to do in a mod.
The best argument against mirrors really is that if the mirror match-up turns out to be boring(again, necron mirrors hi2u), you run the choice of leaving it as it is and thus damaging the game overall, or redesigning some aspect of the faction, which might have result in collateral damage towards the non-mirror matchup. However, both of these things are kind of big "ifs", and depend solely on how well the match-ups are designed.
That being said, if the mirrors turn out to be dumb, I'm completely in favor of removing them, but I don't think mirrors are an inherently bad idea, as some people seem to think.