Windows has a limitation in that it cannot be a member of more than one domain or workgroup at a time. And if you change its membership, it throws the old membership information away. And a user on one domain is not the same user as one with the same username on another domain; they're as separate as if they had two totally different usernames.

So it's not so much a security issue at its base as much as an arbitrary limitation. Of course, some could argue that limiting the computer to one membership might be for security reasons, too, and I wouldn't argue; I'm already thinking about it too much.