reminds me of the remark occasionally heard about Vivaldi's music: he didn't write 600 concertos, he wrote one concerto six hundred times.
I'm currently listening to The Teaching Company's series of lectures on "The Concerto" (http://www.thegreatcourses.com/tgc/courses/course_detail.aspx?cid=7270), and I was intrigued to hear about the degree to which that is, and is not, a fair characterization of Vivaldi.
As it happens, his job required The Red Priest to turn out 2 new concerti per month, for a very long time. So, yes, he recycled a lot of material. On the other hand, he also played a pivotal role in establishing the form of the solo concerto as we know it today, and he was rather more innovative in his composition than most people realize. Of those nearly 600 concerti, there were (at least in the opinion of Dr. Robert Greenberg, the lecturer) probably 20 or 25 genuinely original, distinct, first-rate works. For most composers, that would be a very respectable total -- it's just that they're obscured by the massive volume of more derivative work.
As an aside, I'm enjoying Dr. Greenberg quite a bit. He walks a nice line between informative and entertaining, without ever crossing the line into buffoonery. I learned a lot from the 24-lecture series on "The Symphony", which expanded both my understanding and my tastes.