Hey there,
I actually used to work for an agency that represented film composers about 10 years ago, and have some 'insider" observations if you will. Think of it this way - Gorfaine/Schwartz is a business. They're doing this to make money. The only way they will make money is by taking a percentage of your profit. So, if they can see that you are about to make some serious money, they'll be very interested in representing you. This, however, is less judged by your talent and abilities, and more by your connections and the jobs you have lined up yourself. A lot of people have this idea that agents are out there to get you jobs - it's true to a point, but not completely. The agents are there to negotiate your contracts, get you the best deal and so forth. For big time names, they're more in the business of fielding the offers and helping decide which potential future job would be most beneficial to the composer, both monetarily and credits wise. One or two (or 58) credits on small, low budget independent movies would get you nowhere at Gorfaine, or similar type of agency. One or two credits on movies that made a splash at the boxoffice, with directors who are obviously going places and who are happy to be working with you, will get you in. Makes sense? ;-)
And once you're signed in with the agency, they are absolutely under no obligation to get you work - and that was the thing that was very interesting to observe. You had composers who were thinking "I have an agent, I'm just gonna sit down and wait for my jobs to come in", and they'd be sitting there forever, complaining about the agency, and you had composers who got their own jobs, and had the agency negotiate the deals, etc., and they were just getting busier and busier.
Hope this helps. Cheers.