Well, you have contextual advertising (advertising based on the content of a page) and behavioural advertising (based on the end-user's usage of the site)...
The problem with advertising based on context, is that computers are often unable to understand what we mean, and so therefore will never be 100% accurate.
And the problem with behavioural advertising is that it's plain intrusive, disregards a user's privacy, and can be easily thrown by more than one person using the same machine, in any given period of time, xmas shopping on the home family-shared pc, for example.
Maybe with added semantics, the advertising put in place would seem more relevant to the user, more effective.