I feel they are just like any other business establishments - win win situation for both parties.
It depends on the kind of agency you are tying up with. Some of them have direct relationship with big organizations and they are really helpful. Especially those who are operating on EP placements.
There are some who even charge money for registration from candidates. In most cases they are fraud. Candidates are never supposed to pay anything to recruitment agencies. It's the hiring organizations who pay the recruitment agencies.