Look as if they'd be good for protecting clutch posts in the event of a lowside, but I'm not sure that I'd want to fit one with no cover at all. They've got some serious castellations on them that could easily catch your toe and either do it some serious damage or flick your foot off the footrest when your in the middle of a three figure sweeper.
To be honest, the pressure plate itself is not my cup of tea, I'd prefer one of the DP or MPL ones, but I'd still fit a guard of some kind even if it was just one of those that fits like a collar around the rear of the plate or an STM toast rack one.
As far as weather, dirt and damp. Well, the clutch generates enough of it's own dust so dirt getting in shouldn't be a problem, it would be centrifuged out anyway. The springs might go rusty, but you can get stainless ones. I wouldn't think there are any more issues that with any open clutch cover
[Edited on 22-2-2006 by Jools]