The incentives don’t always work out that way. There comes a point of diminishing returns for the recruiter; trying to get an extra $20k a year for you, might take time away from them closing two or three other candidates. Also, a recruiter does not get any real benefit out of negotiations that involve things other then salary. Extra vacation, bonuses, guaranteed severance packages, etc.

Key take a way from this: never let your recruiter negotiate on your behalf. Once you are at the stage, tell your recruiter to F off and get the best deal you can for yourself.