I understand the logic of helping GoogleBot along, but it's more likely that Google will want to tweak it's AdSense engine rather than pick out manually entered keywords.
Maybe, but there's something to be said for helping them along. Take a page about camper rentals in Europe. If Google doesn't have a relevant ad, it'll probably display an ad for camper rentals in the U.S. or someplace else--which isn't likely to generate money for my European travel site or for Google.
Ideally, I'd be able to specify certain parameters for the site as a whole--e.g., "Europe" and "travel"--so that AdSense would know that the default ad for any page on my site should be an ad about travel in Europe.