there's a theory floating around the ebay forums that if your items are included in express, it hurts your sales on the main ebay site.
many people have reported that their store sales have gone up after opting out of express. I know mine were flatlined, 3 days ago i opted back out, and today i sold something. might just be coincidence, dunno.
several threads about it on ebay stores forums http://forums.ebay.com/db2/forum.jspa?forumID=21&start=0