In general reverting a user folder to a prior version of the OS is a world of hurt. In specific, mixing that pain with the pain of beta / unreleased / unfinished software can really foul things up. A serious effort is expended to make upgrade scripts so that your data can make the jump from 10.6 to 10.7 and there isn't any effort into making that reverse trip.
Once the newer OS touches the preferences, files, databases - it's a one way trip. Even a third party app uses API calls from the OS that can make internal caches and data stores incompatible. There are no scripts or undo on the upgrade to revert these changes and pull out the new items so that they work on the older version of the system. Running both systems will amplify this bad effect: go forward and change things, go back and break, go forward and change broken things, go back and break them in new and novel ways.
At best, you will get a nice warning from the program when it detects that the settings and data from a newer version can't run and the program will gracefully quit. At worst, data will be lost and/or corrupted.
Now - lots of things will still work (mostly) and you will get to learn a ton about how things are stored and how software behaved as it breaks.
If your goal is to learn and play, have at it! Seriously - it's a great learning exercise. Well over half of my most valuable knowledge of systems was gained from seeing them break, poking around to see how exactly it broke, disassembling and understanding the system, attempting to fix, failing and then reinstalling if it can't be fixed. This is a great way to learn what has changed under the hood between 10.6 and 10.7 preview assuming nothing will ever be used again and you don't mind walking away from the machine by erasing it and starting fresh.
For a machine you want to use for real work and data, it's a really bad idea.