I agree to join the program committee of WINE 2008. Right now I am bidding for papers as member of the PC of ACM-EC 2008. Also I am PC co-chair of the COMSOC workshop.
In computer science in particular, we use this sort of activity as evidence that one is a respected member of the research community in question. (Another standard example is being a journal editor.) Most computer scientists seem to list their current (and often previous) PC work on their web pages. Publications get less prominence.
With regard to the title of this post, I suspect there is a tendancy to overdo it, that is, accept too many of these invitations.