Yes. Broadness is needed to reach a broad market. XB360 is gaining a whole extra branch of content and appeal, and it's just as valid to show this at E3 where people are seeing what the future holds for gaming as it is to show conventional stuff. Plus marketing is about getting your message in as many places as possible. If MS chose E3 and no other marketing avenues, I'd concur that was a mistake, but they are communicating to the existing XB360 user base and other media-savvy gamers all about Kinect, while they'll chase down other demographics through other means.
They have the right to do whatever they want, but then we should call it like it is, and not use the copout Nintendo defense of 'well, it's not for us, let's not judge'. Instead, be honest: 'I don't like what's being offered, I didn't like what was shown'.