If MFA's are appearing as one of 3 or 4 ads in a display and different ads are being rotated on your site, how do you know how well a particular ad is converting?
Also, I have strong content sites and my problem with MFA's is not whether or not they convert well - my problem is I think they frustrate people who click on them and those people - my visitors - will be less likely to click in the future. I've seen a number of people on this board express the same concern.
And then a secondary problem, IMO, is most of the MFA's I see seem to clearly be in violation of AdSense's terms. If AdSense is not going to enforce the terms - then fine - just change the terms to allow everyone to build these type sites.
Maybe I've missed it, but I don't think I've seen many people complain about MFA's based on a low conversion rate.