Well, I'm neither published nor agented, so take my words with a grain of salt or whatever. BUT, in my search for an agent over the past year, which I began because I believed it was the prudent thing to do, I have found that agents want either author/illustrators OR a PB writer with multiple polished, marketable projects that the agent loves in order to sign the author. And, as you already know, there are not that many agents who rep PBs at all. As Ellen mentioned, the PB market is so very, very slow right now that few agents are actually taking on new clients without the WOW factor or the multiple manuscripts. That being said, there are exceptions -- I know of one writer who signed with her agent based on one immediately marketable PB in addition to some sample writing in other genres. And there are others who have signed for whatever reasons.
SO....as Ellen said -- and as Harold Underdown has said -- for PB writers, having an agent to *get* the deal is not necessary by any means. Sure, an agent can get you into otherwise closed houses -- and of course, an agent is invaluable when it comes to contracts, etc. -- but absolutely necessary to get published? Not. At. All.
(Full disclosure -- I am still in the process of seeking representation for my PB writing, so it is something that *I* would like. But again, necessary to get published? Not entirely.)