Dear Curious of Essex
If a user searches for something and is led to a MFA site where an ad brings him closer to what he was really searching for then YES the MFA site has done the user a service. In that case, the MFA site helped him find what he was looking for -- or at least helped him to refine his search (think in terms of Adwords/Adsense arbitrage where you bid on obscure keywords and bring up a page with better targeted, more expensive ads).
Assuming nothing changes then those MFA sites could actually be a good thing. However, things change
The better thing would be for the search engine to either display the ads from your site directly or better yet - bring better targeted search results. Remove the middle-man, remove the unnecessary additional step of the MFA site. Better targeted search results also bring along the necessary step of removing the MFA site from the index. Once higher quality sites are shown, it doesn't make much sense to keep the middle-man in the listing.
Those three steps are however not always done together. Sometimes the MFA sites are removed before the results are cleaned up or before the ads are better targeted. They take it one step at a time
Getting better targeted ads would be relatively easy for Google, but I don't know if they do this (but I'm sure they track it): Just track the times when a user comes in to a MFA site from a search query and exits with a click on Adsense. If they had shown the ad directly (and in the right position), they could have had the full payout (instead of having to give a part to the Adsense publisher).
So.... if you can get a MFA site ranking and can profit from it, perhaps you're doing the users a favor by getting them to the places they wanted to be AND by giving the search engine tips on how to improve their results. By getting your MFA site to rank you'd be helping the search engines to remove MFA sites