I like them off so I can keep really scrub the spokes clean. They really do look so much nicer without them (unless they're shiny, clean and rust-free).

The thing is, if you're passing the bike on to a friend or another person, unless you trust them to keep the bike maintained (I don't trust anyone I pass bikes to to do proper maintenance), I think they should probably stay on; derailers that aren't cared for easily go out of alignment.