Sure, if a good recruiter was better at sorting resumes than one of your developers, they'd add value. hell, even if they were only slightly worse, they'd add some value.
The thing is, to justify current prices, a recruiter would need to be rather a lot better at picking good prospects than your in-house hiring manager, and many times better at sorting resumes than someone who actually does the job you are hiring for. My understanding is that the fee charged is in the tens of thousands of dollars; for that, you had better save me a lot of employee hours, or get me a dramatically better candidate.
In theory, recruiters can save a lot of work by sorting resumes once and then using that sort to give some subset to companies A, B, and C - rather than the sort being repeated at each company.
I agree that it is possible for a recruiter who knew the job well enough and that had multiple clients looking for people in the same field to more efficiently sort the resumes, but even if you could find a sufficiently skilled recruiter, I don't think there is 1/3rd of a year's salary worth of value in that alone. I mean, maybe if the recruiter charged me a week's salary, it'd be worth it for the time I'm saving, but 1/3rd of a year is a lot of money; that's a lot of time I or my people could spend sorting resumes.
I mean, if the recruiter knew more about the position than I did, that'd be different. I mean, this would be very unlikely if I was hiring someone in my own field, but hey, sometimes you need to hire outside your field. I can see using a consulting recruiter for that, but most of that value is teaching me what to look for in an accountant (or whatever I'm hiring for) and they'd need to sit in on the interview. Really, in that case, I'd be looking more for a consultant than a recruiter.
The thing is, to justify current prices, a recruiter would need to be rather a lot better at picking good prospects than your in-house hiring manager, and many times better at sorting resumes than someone who actually does the job you are hiring for. My understanding is that the fee charged is in the tens of thousands of dollars; for that, you had better save me a lot of employee hours, or get me a dramatically better candidate.