Convincing people to use Atahualpa

So often I can convince people I am building sites for to do everything my way except for the most important thing. They insist on using some weird theme that is going to cause trouble. (Last one I did was missing the 'more' tag after the excerpt on all the archive pages-easy enough to fix but I don't think I should have had to). Even after you tell people you can make Atahualpa look almost exactly like the theme they picked they still want 'their' theme. It's frustrating when you know you could do the job in half the time using Atahualpa and they would have a better end result. Has anyone had much success talking people out of their theme choices?