I have a theory that there are two kinds of agents (not counting the huge William Morris-type outfits). The first kind believes there really is talent out there worth finding. She doggedly sifts through piles and piles of poorly-written stuff until she becomes overwhelmed and just gives up. "Not currently seeking new authors" is a common mantra for this type.
The second kind believes there is talent out there, and after sifting through the slush pile, does not give up. Instead, she hires a reader. This reader is usually fresh out of college, armed with an English degree and plenty of attitude. He yearns to make it either as an agent or somewhere in the publishing business. Right away, he attacks the job with gusto.
But something's wrong. He quickly realizes that his future in the business will be dim if he starts recommending stuff to the agent that she doesn't like. After all, what can end a career faster than having your boss think you have no taste?
So this eager reader now becomes a professional rejector. You can see it now, can't you? The agent sticks her head in and says, "Anything worthwhile yet?" To which he replies, "Nothing yet, but I'm still looking!" In fact, he quit looking long ago, instead merely returning the manuscripts unread with form postcards attached.
I've met dozens and dozens of published authors over the years, some of them very famous, and I've always asked them one question: "How did you get your first agent?" I can say that not one of them was united with their first agent through the "carefully-constructed query letter plus sample chapters" routine. Every story was different, but each one involved luck, connections, or a combination of both. In one case, the author married his agent.
I have no doubt that people, maybe people on this forum, got their first agent via the standard query-letter route, but I think there's a message here. In general, agents really don't want to know you if you're unknown.