The original question was weather or not they would be more popular if they were better represented. I think that if GW took the time to better represent them, with both fluff and rules they would be much more popular.
I am not arguing that with a little creativity you can make almost anything into an original army that counts as XYZ, because you can. What I am saying is that I can't take that army to a tournament, or take it to my FLGS to play without endless questions. And if I went go out on a limb and make my own rules, then I need to get everyone's permission to even put them on the table. I don't feel like putting that much effort into an army that will rarely see table time. And from what I've seen on the table, most gamers in my area aren't willing to do it either.
So, if GW were willing to expand the xenos races like they do marines, I believe there would be far more xenos players.