If done in a proper sequence, those are no more inaccurate that doing the questline itself.
That's a very ballsy, contested claim, and even if you knew how to perfectly use the console, it doesn't mean that every single beta tester would, or that they would be able to diagnose wether something was caused by a bug in the DLC or their own improper console usage.
I wouldn't be too quick to judge. Those PC players you speak of do a lot of Beta testing for high quality mods as well as official patches.
EDIT : Myself included.
Again, wonderful that you're such a dilligent beta tester, but a pool of PC beta testers is bound to have a few with a modded setup (perhaps with mods that haven't been properly uninstalled).
In any case, every Xbox 360 is the same, so they are a good choice for testing bugs that aren't related to a specific type of machine. With something like Dawnguard, it might pay to leave all the PC-specific issues until later.
I don't think they want PC users to test it out of the fear that they'll copy the files and give it away to people. They also asked whether or not you use a USB storage device for the X360. Just my thought.
I hadn't even thought of that. Any old fool on the PC could copy the Dawnguard BSA and ESM and put it on dropbox.
Also, I'll add that I've signed up for the 1.4 and 1.6 patch betas because I wanted to play on them early, not because I had the http://www.youtube.com/watch?v=g_y2xy7SAFE#t=2m8s.