As far as the leather cambium saver goes, I don't see a big problem. Leather is naturally water resistant - to a point. Also, water IS a lubricant. It can actually make the rope slide more easily, through the sleeve.
I've never used a house sleeve (It's going to be my next piece of gear though), so I don't know what they look like inside. If they are bare metal, I can imagine a tiny bit of a problem if rust forms. I think the rust would have to be REAL bad to do damage to the rope though. I would also think that you could clean the inside of the house sleeve with a swab - like you would a shotgun barrel.
Finally, like you said, If I was going to climb and it was already raining or rain was forcast, I'd use my ring and strap friction saver to avoid any of this.