I don't see a problem with it at all.  Travelbugs IMHO are not a 'trade' item per se, because they move from cache to cache with a specific purpose (unlike used golf balls, which end up in the drink at the local golf course).  I've dropped by a few places just to pick up a TB to move it around, because I enjoy that aspect of geocaching.  Helping further the TB on its goal doesn't necessarily include the requirement that one must continually trade something to help it along.

Now, a cacher visiting a cache just to pick up a TB is very much helping maintain it.  When visiting a cache, cachers tend to note the condition of the cache, and if the container is degrading or has been plundered, it's usually reported, thus helping to maintain the cache.  As a previous cacher stated, if they held onto a TB for too long for <insert reason here>, and revisited a cache already 'found' to drop it off, that's not a problem.  I've helped several cachers out in finding caches I've already found for a multitude of reasons.  It doesn't take away from THEIR game necessarily, but others may disagree with that approach.  To each his/her own I say.

Brian
Team A.I.