That would not have worked for me, signing up with an agency and hoping they would find a donor... I wanted to see every available pool of donors out there. Some agencies will also ask you what you are looking for and keep an eye out for unlisted donors, without having a contract yet. The agencies know you are probably looking at several others at the same time.
Some people don't want to see a lot of pics or know a lot about their donor or they want their doctor to pick one for them, in those cases a small donor pool is OK.