If you hire a beta tester, then you can reasonably expect them to go in with the intention of finding bugs, and then presenting them to you in a clear and concise format that will allow you to categorize, rank, and eventually fix said bugs. The quality of work is reflected by the cost of the contract.
If you sell a beta and make the buyer your tester, then you can reasonably expect to get a bunch of people who are there with the intention to sight-see, and who will then present you with a long, rambling, and unfocused list of all the things they liked and didn't like, whether or not they are bugs. You then have to waste your time sifting through this to try and figure out if anything is important and/or actionable.
It is basically the equivalent of doing screen tests for a movie. It will give the folks in marketing something to do, but it sure as shit never makes for a better product.