If they are corrupt there is no point in keeping them if they are really old there is no point either as if you restore a real old restore point a lot of things can get missed up because of the new files and settings.
System restore isnt a full system backup. It grabs registry settings and personal files and such. But thats it.
System restore is meant to restore the system after something messes it up, like a program install or windows update. This is why most installers or windows updates does a restore right before they do their work, so you can restore to it right before it tried.
Go back to far then registry keys dont match up to new files installed and such and all of a sudden the system is unstable. So system restore will auto remove old points, and sometimes even remove all of them when it thinks they are all outdated.
I dont know its reasons why, MS hasnt released a single bit of info about it, how it works and what it needs to work. For some reason they have stayed very hush hush about it. Strange really.
Shane