I think that the reason they don't let PC users beta test is that it would inevitably lead to people releasing the content to torrent sites, etc.
I remember seing screenshots of Dawguard running on PC before it was even out of the Xbox beta.
Considering PC users have tools like the console and the editor, they have a better chance to catch bugs that slipped past QA.
It looks like I'll be waiting for the first patch post Dragonborn (v1.9) before buying... The way the beta of Dawnguard was handled made me loose confidence in how this initiative was planned and executed.