The allure of the face hidden behind the mask... and the freedom to hide behind one! A bit like having an avatar. You can pretend to be anyone but will reveal more about your true self in the process.
They certainly get me and in SL I can't resist them... as long as they're not simply an uploaded stolen photo pasted on a prim. I added three new ones to my collection this last week and I can't wait to show them off.