Why do you doubt that? Different platforms. They give the pc a beta before every update. What works for one system doesnt allways work for another. Then again you may be right... who knows.
There are two classes of bugs they will be looking for:
1) Cross-platform bugs - most bugs will be similar on all platforms
2) Platform-specific bugs - some bugs will be particular to a given platform
Frankly, I expect there to be fewer bugs platform-specific because they are working with the same engine as the existing game, whereas I think that most bugs will be cross-platform. They will be, and probably already have been, internally testing the game, and I expect they will consider that sufficient to find any extra bugs in the PC or PS3 versions. In any case, while I hope Bethesda takes into account all the beta feedback, I think that building hype over the release was a big part of this beta test for two reasons. First, they are doing a very short beta test, merely two weeks before their tentative release date. That suggests that they aren't expecting to do substantial changes for the final version. Second, they rolled out 1.6 without attending to bugs found by PC beta testers, which suggests that fixing every bug found in beta tests isn't a priority.
Finally, doing a beta test for paid DLC is very different from doing a beta test for a free update. They have to devote substantial time to reviewing applications, promoting the beta test, choosing testers, and following the beta tester forum. Also, they are giving away the DLC free to the beta testers, so they lose revenue for every tester. For all these reasons, I think it is extremely unlikely that they would do Dawnguard beta tests on other platforms.