The fact of the matter is agents have too many clients to be very effective for any of them but the very top writers. If anything, they fire the agent once they see how little an agent does for them. A spec is rarely ready for the spec market when I first get it, so we often go through a couple drafts with my development execs to get it perfect for the very difficult spec market. I then spend about two weeks prior to the spec talking to over a hundred of my producer friends getting them excited about the spec release. I never slip a spec out to anyone prior to the release, so the morning of the release I usually have about 70 messengers waiting outside my door to race each other back to the studios.
Most agents only send a spec out to one producer at each studio.
That's fine for the agent to make favors off your spec, but not as effective for the writer as sending it to multiple producers at each studio. It vastly increases the urgency with which the producers read and value your spec. It also vastly increases the number of friends you're going to make around town. Once we start to get offers in, I turn the negotiations over to an entertainment attorney who is known for record setting deals. From this point, my client and I just hang out to hear from the attorney.