Robert_Charlton - 8:28 pm on Sep 6, 2012 (gmt 0)
RegDCP - Google probably doesn't throw away anything, but certain signals aren't worth the computational overhead or the noise they might create.
Your points about "properly used meta tags" for information retrieval are good ones. In competitive search, though, meta keyword tags have been used ineptly and improperly for so long that Google probably decided at a certain point that they weren't worth the overhead.
Whether they're used for algorithmic spam detection is debatable, but I'd guess not, as so many people think that meta tags are pie in the sky wish lists that Google would probably be getting noisy spam signals.
That said, in public site review sessions, I have seen Matt himself use meta keywords tags (and comment tags) as indicators of user intent. He almost always looks at them for evidence of keyword stuffing (particularly for the four p's), which suggests that they are looked at in Google spam team manual reviews as well.