This is something that I have been thinking about for a while. I also saw a similar post on /r/geocaching a few days ago.
I have run into a couple wet logs. On one occasion the container appeared to be OK, so I segregated the wet log into a new ziplock and placed a new log in another ziplock. That way if the owner wanted it for some reason, it was still there. Another one had only room for one, so I trashed the ruined one and replaced it. I would feel a little bad, but the cache had a maintenance request for over 2 months.
Lastly I found one the other day in a nice park and the whole thing was nasty and ruined. Didn't feel right, so I left it and reported it. It's apparently been reported for about 5 months. :|
I think I have come to the decision to add a log if I can, but not remove anything. I don't want to be 'that guy'.