First of all, what would qualify someone to be able to give a valid reveiw of an agency? How many times would he have had to have posted a review of one of their girls and over what period of time? If someone has never posted a SP review, should he be allowed to post an agency review? Someone who deals with a particular agency on a regular basis over a long period of time may have a different point of view than someone who has dealt with them once or twice in the same week.
Could agency reviews be useful? Maybe, but only if they were posted by experienced hobbyists who are respected on the board and have contributed a decent number of SP reviews from a variety of agencies to establish their credibility. Just saying that you have been around for many years and have seen X number of girls means nothing if the reviews aren't on the board to back up the claims.
As MERB stands at this moment, I think that anyone who wants to get a valid picture of an agency only has to do a search for the agency name and they will find tons of information in the various threads and reviews that are already on the board.