I think beta releases are generally fairly stable - we don't usually make significant engine changes any more, so usually there are only more minor code changes or new features being added. However every now and then something important does get broken. I think it's still fairly low risk though - as long as you back up your project just before installing a beta, if it doesn't work you can go back to the previous version and still open your project. Usually within a week or so a new beta is out which fixes whatever was broken.
We haven't had any "corrupted project" bugs for a long time now (although never say never...) but even if that worst case scenario happened, you have your backups (right?)
The main purpose of beta releases is so if we do break something important it doesn't affect everyone - only those who have opted in to help testing.