Theoretically, a Beta Test distribution is software which is new, or substantially different from its predecessor, going to people who are willing to endure a little uncertainty and possible instability to get an opportunity to influence the development of that product. The Alpha Test is conducted in-house to expose the most obvious bugs, which are usually corrected before Beta, or at least noted in the Read Me as known problems which are being worked on, and aren't fatal.
Betas carry no guarantees.
If you run a third-party extension which, combined with the Beta software, makes mincemeat of your hard disk data, you'd better have a backup, because you've got no recourse. Those are the kinds of problems Beta Tests are intended to uncover. No developer can expect to be able to test their products comprehensively before release. There are hundreds of Mac hardware configurations, and a virtually unlimited number of possible software configurations living on that hardware. Betas hope to get the product onto as many of those configurations as possible, and they especially hope everything goes smoothly on all of them.