Don't get me wrong, most recruiters are worthless and add little to no value.
Recruiting firms have guarantee clauses in their client agreements - meaning if a placed candidate ceases to be employed (referred to as a 'fall off' in the industry) for any reasons in a specific time period (typically 3 months) the recruiter must conduct the search again at no additional cost to the client.
Moreover, if a recruiter has more than a few candidates fall off, most clients will take their business elsewhere.
My personal experience with applying through recruiters is it puts you at a disadvantage right from the get go. Most hiring managers believe applicants that come through recruiters are more likely than not to be of low quality, so right off the bat their first impression is suspicion and skepticism. On top of that, if they hire the candidate they have to pay the recruiter a percentage on top of what they pay the new hire. If you're not exceptional in every way they could possibly imagine, you're not getting the job.
Of course, there are exceptions. If a company has worked with an exceptional recruiter in the past and trusts him or her, then things are different. I have yet to meet even one recruiter that fits that description, though.
Experience 1: 25 years old and applying for a $200k/year job. I didn't get it :( but the recruiter had my back and significant sway with the employer. He filled around 10 roles per year. He was also buddy of the person I sat next to but this was unknown to me, I was cold-called for the role. "This is small-fry for me, I usually just fill $500k+ roles."
Experience 2: Recently (10 years later) a recruiter asked me my expectation "It's in your interest to get me as much as possible" was my response. I _know_ there's more to a job than financial package, but that was that question dealt with. She called me after each interview within the hour, and gave me status updates every 2 days over a period of 4 weeks. Happened to be in the same city on a trip, and had lunch together. Nice lady. Didn't take the role in the end. Still in contact, no bridges burned.
I also agree somewhat.
Experience 3: I sit next to an HR Ops team that staff large tech companies. A team of 20 people that download and forward CVs all day. The level of technical questioning before forwarding a CV is "How is your XXX language" and that's that. Applicants are sorted and pigeon holed into buckets depending on existing role and package, and no more. They have no bargaining power with a client (on either side) as they simply don't understand anything related to the role. Their KPI is how many roles filled, that's all, and when the client gets unhappy they email the GM.
So, in my opinion, it's mixed. But some headhunters really are good. Differentiate yourself, make yourself special, then the good headhunters will seek you out.