I know this will largely be a personal thing, but it's interesting to hear peoples input. I bought a Rebel XS to get myself moved over to Canon from Sony. I then got a 40D. I originally thought the XS would be nice to have as a smaller and light weight alternative to the 40D for travel, but I then realized I would probably just bring the 40D with me. So at this point it is a second body for backup, and two body shooting. I haven't yet used it as a second body (with another lens) but I think it could be very useful in some settings (like when I'm helping out at a wedding this summer.) So far, for the month or so I've had the 40D, the Rebel has sat in a cabinet.
But I'm starting to think that at this point, the $350 or so I could probably sell the body for might be worth it to spend on glass. My wife and I have a new baby and not a lot of spending money. I'd probably try to sell the XS plus kit lens for the Tamron 17-50, or just the body for a Canon 85/1.8.
Any feelings on this?