Other people apparently don't have this feeling at all. Maybe I shouldn't have been surprised by this, but I've definitely been caught off guard by it.
Just like with email spam I would expect that a big part of the issue is that it only takes a minority of shameless people to create a ton of contribution spam. Unlike email spam these people actually want their contributions to be tied to their personal reputation. Which in theory means that it should be easier to identify and isolate them.