Though I ultimately didn't take their offer because I was able to play it against a local company for a matching salary and bonus to avoid a move it was a good experience working with them and only mildly painful to break it to them that I would be taking another offer.
An extra 5% on $100k to you is $5000 but for the agent it's $250. They are not aligned to get the best price.
It doesn't mean they won't try hard but keep in mind $5000 means more to you than 250 will to a company.
I wind up handling negotiation past the intro and initial position details myself anyway it's something I find best done by me as primary concerned party and apparently so very few people negotiate at all so there's usually wiggle room.
That's why I wound up using the offer they found me to push one I liked more locally because you're right they're only so aligned with me and being able to fall back on that offer let me likely top out the comp range at the local place.
The big recruiters have a continuous churn of entry level employees, so reputation does not play a part in their mission. Maybe this kind of relationship between 10x developers and representation would be beneficial for the entire industry.
I'll bet $1000 the same would apply to tech.
Most people have no idea what they are worth - which usually means undervaluing themselves, but just as bad, overvaluing.
Having someone who is objective and understands business just look out for you a little bit is probably worth 5% of your income. You'd earn more than that in the end. Esp if they had easy access to lawyers, or could mostly speak the language, etc.