The recruiter is always out to make as much money as possible. Recruiters typically operate in one of two modes:
They might be working with a company specifically, in which case they can try and find the best possible candidate, and may lie in order to lure that candidate away from his current job, or other offers.
They might be competing against other recruiters to fill a position, in which case they will do anything possible to push their candidate ahead of the competition's (and thus get paid)
Both cases can be detrimental to you, the candidate.
Working With a Specific Company
In this situation the recruiter will typically be willing to represent the company in a better way than it deserves in order to get you to come in for an interview.
Yes, you might walk away if you catch them in a lie, however many candidates can be bullied, or scared into not inquiring about every aspect of a job.
For example, when I took my current job the recruiter promised me that working from home was a common occurrence. However, he also said that this privilege is not accorded to new team members, and that my asking about it would be misconstrued as being more interested in working from home than caring about the company. To my chagrin, I did not bring this up in the interview. By the time I found out it was a complete lie, and that no one worked from home, ever, I was 2 weeks into the job. It taught me a valuable lesson.
Competing Against One Another
In this situation you don't know whether a candidate is being serious about your chances to get the job, or whether they're simply desperate to get their foot in door with a certain company by simply sending them someone.
I've had recruiters encourage me to modify my resume in order to basically lie about my qualifications. I refused, and never heard from them again.
I have also had recruiters lie about my qualifications to potential employers, something which came out in the interview, and revealed the whole experience as a giant waste of time.
These sort of recruiters simply care that they get their commission, not that you enjoy your job, or that the company gets the best possible employee.
I'm sure they exist. I just haven't run into one yet.