I guess I just have an easier time divorcing the crunch from the fluff maybe? Or maybe since I have played Tau for so long, I deal with any color Space Marine the same way: Run away and shoot them. :P
The reason I, and I think most people, dislike proxying is just because it is confusing. It isn't the same (to me) when it is just a color swap. If you tell me you are using Vanilla, SW, BA, or CSM, I can remember that because it is across the whole army. Granted this is probably different for everyone. This is the same reason I find swapping marine books better than swapping other books. It is just easier.
I know there are some other really good armies out there that are counts-as, but it is a lot of effort to get that to work well the more different the armies are. Using Necrons to represent Eldar would be much more difficult, for example. Eldar swaps might be pretty easy too. I still toy with the idea/dream of using the DE codex to make a Kroot army.