Personally, I watch every episode, because sometimes they surprise me. For example, the Anko arc. I thought that was a good arc. Had it's flaws, but overall, a decent arc. Sometimes, certain parts of a particular bad arc had it's small moments.
I think people keep watching, because of hope. They hope that it will redeem itself and be rewarding. However, as episodes of dissapointment go by, they start to lose more and more hope, thus them saying they won't watch anymore. There are already a number of people that have stopped watching and won't until the fillers end. But there are still some fans who watch being hopeful. Others watch because they find how bad it is amusing. And others just like inflicting pain upon themselves but find cutting their wrists a bit too drastic XD.
Plus, it's like a train wreck. So horrific, you can't look away.