Not sure about that. For advertising I'd agree only show a Natal system, but having the peripheral out for existing XB360 to catapult the user base and word-of-mouth would be a good thing, rather than leaving the system to compete with Wii for some months before leveraging the existing userbase.
Delaying the accessory by about 6 months would be sufficient to let it grow legs on its own.
Ads, media coverage and spot coverage on late shows, talk shows etc would all have enough time with 6 months. After this exclusive period, an add-on introduction would make sense.
But it absolutely needs to have it's own identity. Having it run on an xb360 will have potential customers associating it with that product and it's well established image as a "gamers" device and all that entails.
Granted, the userbase may not grow with traditional gamers as fast, but that is 100% ok, because that isn't where this system is targeted.
In fact, they'd likely see cross adoption of existing xb360 gamers buying the new natal package to take advantage of the smaller design that hopefully has a lower running dB level and less likely rrod.
Don't forget, the wii had no problem coming out the gate with very brisk sales. With this hardware also being xb360 compatible (ie the same darn arch in a new box) they'd see a pickup of sales for natal, but also potential new cross-over customers to traditional gaming ... and new live customers of course
It makes sense timing wise too. 2005 - 2010. A good 5 year run on the design. Likely will have the ability to drop the core price down to $150 by then too. Assuming natal is $200, they could pack it in the box for roughly $350. As a "new console" introductory price, that's fairly cheap. Especially when compared to Wii. Consumers will see a system that not only tracks a waggle, but their entire body... and as a nice bonus, the games on this natal system look a heck of a lot better than wii ... even on their new hdtv. Heck, it even plays their dvd's.
The fact that there are millions of users already on live ensures that when they want to play their casual games in social settings without anyone there at the time, they will be able to do so relatively quickly when the add-on becomes available to existing xb360 users.
Of course, you and I and everyone "in the know" will know the deal that this natal 360 is really an xb360, but to those outside this gaming realm, this is a new console that does what wii does, but better ... and isn't much more money.
Biggest reason IMO to have this system as a separate platform is for developer support. When developers can look and see a platform that is growing rapidly (I'm going to guess half the rate that Wii grew and say 250k per month) and has a great dev environment and a partner in MS that from what I can tell, has been very good working with 3rd party devs ... it spells success.
Wii success, no ... but Natal360 + xb360 sales could give Wii a run for the money.