Cons: chain tubes are noisy, messy, and hard to clean
Pros: they keep the bike cleaner, and possibly your legs as well (depending on the bike)

My Seiran came with a choice of with/without chain tubes. The tester had tubes, and I found them annoying. I ordered mine without -- much quieter!

Careful: if you go without tubes, make sure nothing touches your chain. In this picture (http://bikezen.org/pix/seiran/target2.html) you can see a cable getting too close to the idler. You also see my equipment bag rather close to the chain. (Both problems were remedied...) Chains and idlers are very unforgiving to cables and such.