I've taken my olympus XA on a few trips and I think it's a must have, even if it's as a 2nd camera. When I travel I try to go as light as possible because you never know how long you'll be walking around with your backpack. The XA is a great camera with a great lens, and it fits in the front pocket of your jeans. Plus, because it's aperture priority, it's a great camera to take out at night. I've gotten some nice shots just setting it on a table top and letting it take a 1-3 second exposure. And since you can get one for less than $50 on ebay there's really no reason not to take one as a backup.
On my next trip, though, I'll be taking my new leica cl as my main camera. I've only had it for a week, but it seems like a great travel camera if you want to be able to change lenses and go fully manual. I can't wait to take it to southeast asia. 🙂
Also, I bought an olympus RC for my sister a while ago and would recommend that as well. It's one of the smallest fixed lens rangefinders, can be used in full manual mode (although without the meter 🙁 ), and has a really impressive lens. When I shot a roll through it I was blown away with how sharp and nicely contrasty the photos were. I was able to pick one up for $70.
Hope that helps.
B