I have in the past - mostly I figure that the small time spammers are easy to get around - the big ones that do redirects and nasty shit get reported - one word of caution though - best to be extremely clean yourself if you do it - it's not too hard for Google to figure out who you are and what sites are yours when you report, as they tend to assume that you are reporting someone above you
A secondary concern that I have personally is that there are some techniques that will put me where I want to be in the SERPs that might be considered spam by someone that doesn't understand the techniques - I'd hate to get reported just because someone "thinks" they should be above me in the rankings - a hand review of a site by Google might cause someone who is deserving of their ranking to be taken down as I think someone on this board had happen to them in the past
that said, there are some techniques for ranking high that some would consider spam, but are just methods that work well with Googles algo right now and I don't think I would report those as spam as I might try them as well
Comes down to a personal decision of what is spam? Unfortunately some people have a very strict definition of it and some are looser so it's open for interpretation in most cases
Google defines what is acceptable and not on their WM pages, but that is what they want in an idealistic situation - should we give up profit to help Google? Im not sure
