I love the idea that with both FF and APS-C bodies, most of my lenses will have dual personalities.
DA21 on APS-C and FA31 on FF will give the same field of view, but will offer choices between speed, flare resistance, compactness and bokeh. Sweet!
Want a normal lens? FA31 or DA35 macro on APS-C, FA43, A50/1.2 or DA*55 on FF. So much choice!
Macro? 35 limited on APS-C if you really want to rub your nose in it, DFA100 on either depending on how creepy the crawlies are, and bung it on the teleconverter if you're properly arachnophobic.
Compact travel kit? Leave the FF at home and pack the 15, 21, 43 and 77 limiteds. Wildlife a priority? WR long lenses on APS-C every time.
Low light concert or serious landscape trip? FF all the way.
No way I'm not having both