I have not taken part in any Beta testing as of yet, so I might be completely wrong here.
The concept of having the public testing your game before it is released to even more members of the public, is a good one. Diversifying your sample group from a few play-testers to many random people spread across the globe, might make the "ironing out" process a lot quicker and more efficient. With diverse players, comes diverse game play styles and approaches, that might highlight a few things that would otherwise have been overlooked.
The distribution of these Beta keys is where I do not necessarily agree. Of course, the main aim of video game publishers is to make money (the developers are a different story, since they try to make the best game they possibly can). It is the role of the publisher to sell that game and try to make whatever money they can out of it. So pairing an expensive
edition of a game up with a Beta key for a much anticipated sister-game is the perfect solution for them, since they get to make that money there.
I have to say that this route is not necessarily bad if the person buying the limited edition of a game actually intended to do so in the first place.