Mark,
The biggest reason for me to model a picture frame in SU is for design. I can get a feel for the frame proportions (thickness & width of frame, mat size & color, etc) and general wood color/species. I import the picture, or a picture of the artwork, then stretch it to size in SU. All of this gets me an accurate depiction of what the picture or artwork will look like so I can get approval from the client (my wife). And when for instance I just dropped $100 on Wenge for a rather large frame, I don't want to screw up a dimension in the shop because I didn't take the time to design it correctly.
For me, it's organic to go from the computer, to working drawings, to the shop. It's also pretty quick since I think I'm rather proficient at SU. If you can't rationalize it, to each his own.