For me, recruiters are useful for two reasons: to find me interesting opportunities and to help me get an interview.

When they've served those two functions, I'd prefer them to just get out of my way. From that point on, I feel my own interests are best served by me representing myself.

The recruiter's real customer is not really any one particular candidate, who they might see just once in their whole career, but the employer, who will likely be a repeat customer if the recruiter satisfies their needs. Looked upon from that perspective, it's not always in the recruiter's interest to land the candidate the highest salary possible. That might get the recruiter a higher cut that one time, but their real customer might not be too pleased if the recruiter keeps sending them expensive candidates. They'd be far happier if the recruiter managed to get them a sweet deal on a good worker.

So I'll just keep doing my own negotiations, thank you very much.