Contextual ads are closer to display ads that to search ads. Forrester Research decided
to break down status quo and removed contextual ads from the search bucket.
Here is the reason, why they did it:
I would add one other reason.
Role of contextual ad is to back up the search process. Provide more information about a product or service.
Contextual ads are often more branding type of advertising, than direct response advertising.
This is the reason they are closer to display advertising.
Contextual ads should have 26% share from display advertising in 2010.
Here is the Forrester`s forecast from 2009 till 2014.
Forrester`s – US Interactive Marketing Forecast, 2009 to 2014 can be found here.