Funny, I just talked to my mechanic about this subject just before we got rain in LA.
We were looking at an old beat-up M900 with a vented clutch cover. The pressure plate was just awfully dirty and I think it was stock. He basically said there is nothing to worry about. Rain, snow and even some sand getting into your vented clutch won't effect the engine in any major way. I hear the bikes in the Paris-Dakar Rally have vented clutch covers.