Think about ties that bind. Like an electronic shotgun, a resume distribution service sends your resume in one digital swoop to thousands of recruiters' e-mailboxes. As Napoleon said, ability is of little account without opportunity. What RDSs do for you is provide the opportunity for visibility. Try an RDS if you know or don't care where your resume is sent, if you recognize that your resume will become immortal in cyberspace and if you realize your resume is likely to be trashed unless the usually overwhelmed recruiter zaps such documents directly into a searchable database.
Specialization is usually far better for job seekers.
You can't control the third issue of overwhelmed recruiters, who understandably lament the number of resumes e-mailed to them that they can't use for the kinds of jobs they're trying to fill. Finding their e-mailboxes bursting with unsolicited resumes to wade through one by one is not a task they happily face. RDSs make it tap again and again.