> In programming, there is a hope and expectation that people will last substantially longer in a job. Therefore you do not need to go back to the hiring well nearly as often, so it does not make sense for you to have a dedicated person who is going to regularly try to hire you out.

In practice, that's seldom possible. Hell, we're on hacker news - it's a tossup as to whether most of the companies you hear about on here will still be operating in two years. If those are the companies you work for, you'd be best off with an agent.