What makes frames less desirable than seat posts, may I ask? (I guess if seat posts are steel and the frame is aluminum?)
Common wisdom is a seatpost gets you about $10 which helps towards one's daily fix.
Actually bike theft around here falls into two categories -- high end that are shipped out of area quickly and low end which are chopped up and "recycled" for a quick buck.
Lots of the chop shops have been found of late due to a community effort to try and clean things up.
1. frames cost more so harder to flip
2. frames typically have a serial number (equivalency of a VIN) which make them trackable for theft.