< note - just so nobody gets confused. I assume you mean Adwords rather than Adsense >
WMT does have some data accuracy problems - always has to my memory. But I still find the WMT information to be a very useful input. It is, for example, it's the ONLY way you're going to see anything about the number of search impressions you're website is getting.
It's one Google tool where they are giving you their information, rather than the other way around.
I think the site: search is obviously something that an average visitor will not do and is either a very sophisticated user or more likely a webmaster. It makes sense then that G will target this user because then it can push this user to start using AdWords.