While it would have been great to see a transgender person play this role, is this not the same as saying only straight people should play straight roles? That only gays should play gay roles? Only lesbians should play lesbian roles? If this were the case, Neil Patrick Harris would not have been able to play his wildly popular role of the womanizing, disgusting pig, yet amazingly interesting role of Barney Stinson in How I Met Your Mother. Correct me if I am wrong on this, but I do believe there is a double standard going on here.
Disclaimer: I have not seen Dallas Buyers Club so I am not vouching for his acting abilities in any way (good or bad), rather just stating a something that I believe to be true.