Blizzard DOES use the beta for hype. With SC2 they gave beta keys to preorders to help generate revenue and interest prior to launch. Not a lot of participants actually know the reason for a beta and actually use it as an excuse to play the game early but Blizzard already knows this.
Blizzard's job of releasing a public/invite beta is to test:
A) How the game functions on the boat-load of machine configurations available. When you sign up for beta invite on battle.net you need to create a "profile" of yourself by uploading your PC specs. Blizzard has always provided games that can run on a low end machine with features and graphical tweaks that still allow it to look good on a new machine. For me SC2 had to run on low/medium on my 2 year old laptop but with my new PC it runs on Ultra with great effects!
B) Network stability. B.net2.0 is rather new and they need to test the matching functionality as well as load test the servers for every reason under the sun. Having willing participants allows them to hit all these goals while still generating the hype.