And yet they are our only weapon against business owners who are just as clueless but lack the incentive to pay a competitive wage. When it comes to salary negotiation, it's nice to have Shithead in your corner.
So no, your interests really aren't aligned, and they will screw candidates on salary negotiation, take the slight hit on commission, and then make it up in volume.
I've had bad ones of course, I think everyone has. But the good ones can do a lot more leg work than you have time for and find you a much better situation than you could find on your own, plus play both sides to make their comission as big as possible which benefits you.
The best are agents, the worst are pimps.
Many companies are now sharing some of the recruiter costs with the employee if they go direct. In London [currently] it is an employee market, so this is a simple and effective incentive.