They can be a 2 edged weapon, they take up time, but can also boost your own interest.
For me, projects are a bit less public these days, just being sensible - quite a difference to go from a tutorial section covering a game, to nothing at all.
One thing with WIP posts though, is they can put on the pressure when it's not really necessary or welcome. People see a nice looking game and proclaim it to be the best thing since sliced ham. That puts pressure on the creator, it becomes daunting to meet those expectations, which tend to be formulated before the game is even at demo stage. If people just showed support without getting all dewy eyed, then I think there would be less pressure. I've seen amazing WIP games die with all the positive feedback imaginable. Personally I can't help but suggest things in WIP threads, which I hope people see as a compliment - I mean if your WIP thread gets people thinking and coming up with ideas, then your onto something, surely.
I think that the best WIP thread would be at a decent demo stage, where your ready to get proper feedback, can post YouTube videos and have some finished media. Folk seem to appreciate when WIP threads are started quite late in development, when there's lots to see and maybe play the game as well. People like to give feedback to projects that will be completed, and maybe even have their feedback positively influence the game, get rid of a bug or improve an effect, something like that. They feel part of it and will support the project vehemently.