The engineers doing the interviews certainly have no financial incentives either way... Recruiters make contact and did a preliminary "do you have a pulse" conversation, and then shepherd the process, but don't have input into the decisions along the way. Their incentive is thus to find great candidates who they think will make it through the process.
I think that depends if you're talking about internal or external. At least everywhere I've worked our in house recruiters don't get paid a bonus when they hire someone, it just their job. If you're working with a recruiter that doesn't work for the company hiring you that person may have a more direct financial stake in your employment.
Consider their financial incentives first before ascribing some altruistic motives.