If the goal is to get the hiring manager the right person, then the best resumes should be submitted without imposing some arbitrary diversity requirement.
I can see value in being offering 3 different 'types' of candidates instead of 3 different candidates of the same type, even if the latter 3 in theory have the strongest resumes.
I take the point but by their own admission, resume acceptance is at 50% when it could be at 80% so on that score they are failing to convince the client of the benefit of that diversity.
If I am a hiring manager and I am rejecting 1/2 resumes as opposed to 1/4, I'd question what the search firm is doing.
I can see value in being offering 3 different 'types' of candidates instead of 3 different candidates of the same type, even if the latter 3 in theory have the strongest resumes.