I'm not a framebuilder, but I am a mixte fan and I like the look and the lower stepover of the curved diagonal tube.
unterhausen brings up a great point. I have noticed that when a frame has a curved diagonal tube, it sometimes terminates at the drop out, as with a straight diagonal, and sometimes it ends intersecting the seat tube, above the dropout. I think that with perhaps more bend that the diagonal is lower where it passes the chain and this upper landing point assures that there will be no rubbing. It looks good too with the deeper bend.
This frame from Muse Cycles demonstrates that deeper bend intersecting with the seat tube above the drop out.
Muse Cycles Mixte