WHO NEEDS AN AGENT?

So now we come to a question: who needs an agent? A writer friend (also published) and I were discussing this question last week. He, like me, has gotten as far as being published, getting a web site, etc., without third-party representation, and, like me, is happy with the way his publisher, which I share, has treated him. But as his career (and hopefully my own) as a writer advances, at what point does it become prudent to have an agent whose primary interest is in the writer's success FIRST and the publisher SECOND? A primary question might be, is the contract I now have, or that has been presented to me, a fair one, or is this just the best I can expect to get as an unknown author? Am I expected to take on promotional responsibilities that I am not equipped to assume, oar that can and should be handled by the publisher (who, after all, benefits more than I do if the book succeeds)?

Certainly a question worth considering.