Well actually, Zimmer has a studio with like 40 other composers working for him, all he does nowdays is come up with a few themes and harmonies and hand it down to a young noob fresh from the academy to orchestrate it for him. Danny Elfman could not read music and when he got his first gig as a composer in hollywood he said he has spent two months locked in his flat working on a score. He was composing, learning how to read and write music and transcribing at the same time. He had a friend that could read music helping him, but still... thats a lot of work. But you can still find articles and intreviews on the web about other composers in the industry calling him a "whistler" (a slang for a composer who can't read or write music so he has to whistle his ideas to other musicians or transcribers). Something similar with Zimmer too.