I can think of a really good one: he spent the money on it and if he would like it better that way then he should do it.

Not meaning to pick on Carleton here, but it seems like this is a popular sentiment. I understand the whole "it's a nice bike, and it's good to have a geared bike sometimes, etc. etc.," but at the end of the day, if he likes the frame and will get more use or enjoyment out of it as a FG or SS, then I see no reason not to do it.

Worst case scenario someone thinks you're a jackass for riding a conversion that should have stayed geared, but if you're too busy whipping around enjoying your bike you won't care anyways.