If a beta build damages your Vox we will ensure you are helped. We are asking for your help, so we wouldn't leave you high and dry.
As for incentive...as with the other beta programs being run with volunteers here at MR, the incentive is to help us improve your favourite eReader and get your feedback favourably prioritized in our dev list
As for rooting, as long as we know what you've done to your Vox I think it's OK. We need to understand the conditions under which people are encountering issues.
James