I have had the same problem with my mower. I have not had to replace it as many times as you but that plastic pinion gear strips pretty easy. I also know of two other people who have Craftsman riding lawnmowers and both of them say that the only problem they have ever had with them is also that starter gear. So its not just you, and it sounds like its something Sears needs to look into.
Richard Beall
September 2005