Here's one good reason to use appropriate META tags:
I run Google Adsense banners on my site. Without the META keyword and description tags present, Google doesn't do a very good job of serving ads that match the subject matter of my site. I keep getting stupid MMORPG and dating ads, among other stupidities (despite the fact that I explicitly removed those from the Adsense backend). With proper META keyword and description tags present, Google tends to serve ads which are more relevant to our subject matter, in this case, photography. This makes it more likely that my users will be presented with ads that pique their interest, and the chance of them clicking on those ads is much greater.
I've run many tests to confirm this, both in regards to the ads served and the likelihood that they'll get clicked. Having proper META tags present is definitely a huge benefit in this regard. With the tags present, our Adsense revenue more than doubles.