How often have you searched Google and been sent to one of those pages?

They don't want useless content in their serps. If someone goes directly to the domain, that's in no way related to search quality.

It is very related. Google made span sites and splogs profitable (through their various add networks). Therefore they are the major funder of spam sites- even if they themselves do not promote them.

+1 and my perfectly non-spammy blog was rejected by them. They didn't mention the reason but I have a hunch that it was because it isn't about any one or two things. It's about a bunch of things I find interesting - both personally as well as professionally. Now, google must have found it difficult to determine what ads to show, so they just black-listed my blog for ad-puposes. Period.