My opinion is that while there is any risk in a trade it should be feedbackable. Very few people use middlemen for less then 20$ transactions so usually the risks is worthy of a feedback. However I have an issue with Verified Middleman being redundant. I feel there should be more users with the rank, people such as @jackshow & @Muffins are, in my opinion, worthy of the verification however they do not meet the requirements (not counting up all their FB). Maybe the rank should be given at @Maldesto's, @Anne's and @Dex's discretion.  
  
But how many current middlemen would give up their time for nothing? 
  
I feel people see the FB as a form of payment that's why they're not charging. But your point is valid.