It's a good point being made, I just don't think you're trying hard enough. Maybe?
Space marines are one of the oldest races around, GW does put a lot of effort into them and they have a very well developed backstory. Xenos on the other hand tend towards a blank slate, this can actually provide you with more possibillities than you would otherwise have with space marines.
Don't get me wrong, i agree that you have a lot more scope with xenos and if you put a lot of effort in you'll get a great final result - but one of the points i was trying to make is it's a lot harder to get your vision across with xenos. The execution of modelling, painting and fluff really needs to be spot on with xenos to get the same effect as an SM army.
If i want to make a highly mobile, desert themed army I just use BA or RG as my primogenitor, paint them a sandy colour and call them the desert rats or something equally stupid. Just from the name and mentioning the parent chapter i can convey a massive amount about the army.
If i wanted to do that with Tau i need to do a hell of a lot of modelling work to get my idea across without an accompanying A4 page of fan fiction.