I also used to hang on to prompt books for some time. I then reached the point where, as others have stated, the producers/theatre started keeping the book in their archives. Storage space also became an issue, with moving around for a bit, and then settling in NY, I just didn't have the space and energy to keep track of all those binders, and my parents didn't really like having boxes of useless scripts in their basement either. In the past six years, basically since becoming Equity, I have only saved one script (besides the grad school shows, since I knew they would want to see those for portfolio reviews, after which they too got recycled). That was for the show I did for the Fringe last year, because the producers asked me to hang on to it as they were actively pursuing another run for the show (which will actually be happening soon, rehearsals start on the 21st).