I've worked with third-party recruiters for many jobs I had while contracting. The pitch you mentioned can work, but only in the high-turnover world of contract consulting for big companies.
Big companies have a specific reason for going with contract employees. It gives them (the company) a legal separation from the person who would otherwise have been an employee. That person can be unceremoniously dumped out onto the street if things aren't working out, and the company is willing to pay extra for that option.
Startups can't afford to pay a 30% premium on technical help just so it's less risky to fire the bad apples. And anyway they aren't big fat targets for lawyers like big companies are - good luck finding a lawyer to take your case on contingency when you're suing a company that's having trouble making payroll.
Big companies have a specific reason for going with contract employees. It gives them (the company) a legal separation from the person who would otherwise have been an employee. That person can be unceremoniously dumped out onto the street if things aren't working out, and the company is willing to pay extra for that option.
Startups can't afford to pay a 30% premium on technical help just so it's less risky to fire the bad apples. And anyway they aren't big fat targets for lawyers like big companies are - good luck finding a lawyer to take your case on contingency when you're suing a company that's having trouble making payroll.