You simply don't want to run a beta, especially the first beta of any OS X on your main machine.
Given the OP's reliance upon his/her machine for life-crucial exams, I will agree with you, however, I would like to point out a few things:
1) This isn't even a beta. The build that Apple were running at WWDC, most likely is CLOSE to beta, and several stages ahead of the one we are provided.
2) If you read through the bug-report thread in this forum, you'll find that none of the common problems, are machine-breaking, or even problematic.
3) The BEST way to help Apple, and to gain personal knowledge about Mac OS X and how it works, is to install 'first betas' on your main machine. On a main machine, people commonly keep it up to date and free of malware. At least using your main machine, even on a newly created partition, eliminates these as a potential cause of problems.
Finally: Installing a DEVELOPER PREVIEW, especially the FIRST developer preview, is a chance that a fair percentage, if not the majority, of people in this forum, have taken, knowing the risks.
If someone who has never experienced an early Mac OS build, comes in here and asks a question regarding reliability, stability or whether or not an app works, or whether they can do this or that, the morally correct thing to do, is to help them in making a decision or assisting them in solving their problem, and not to be making a carte-blanche statement, whilst mildly insulting everyone else, for example this statement:
and not just that you can text with green bubble people.
Now that's just not helpful to anybody.
The point of this forum, and the first developer preview build, is to help Apple iron out crucial issues as soon as possible, and to help others do the same, or, at the very least, help them improve their knowledge and understanding of Mac OS X, if they CHOOSE to accept the risk of installing a rudimentary build of an unreleased operating system.
Take that same response into a Linux distro forum, and the responses you'll get will be far less mild-mannered, and far-less tame, than the one I have just given you.