I would phrase some of this differently.
First, if one has to ask this question, then the answer is no. If one is not experienced enough to know the risks and how to deal with disasters then beta testing is not for that person. But I disagree that part of beta testing isn't early access. Beta testers are unpaid workers helping Apple develop their products. In any arrangement there should be some form of quid pro quo. Early access is a form of reward.
As to the expiration date I would be surprised if it is set to be before their expected release date. If it were, then machines that are not upgraded (perhaps because the beta tester lost interest) would be hosed. That would just be Apple arbitrarily abusing people who had volunteered to help them and that's not good policy. If one were feeling brave, one could disconnect his computer from the Internet, manually set the date to Dec 1, 2014, and try to reboot. Of course this is like pulling the trigger to see if the gun is loaded.
I agree that the Public Beta is a better vehicle for the idly curious.