I always thought it was obvious why Microsoft chose to include an HD with the Xbox and, now, the Xbox 360. Xbox Live. They spent mega-bucks setting up online service and, at least to me, it seemed they were always pushing it into the minds of developers and consumers alike. It was a way to differentiate themselves from Sony and Nintendo and get to the forefront of online gaming (as it seems that's where gaming is headed).
Now, if there had been alot of online games for Xbox that offered downloadable content and such then it's safe to say that a memory card wouldn't have cut it. A massive storage device was the solution. Also, having the HD with the console from the get-go is an incentive for developers to utilize it, and consumers to jump onto Xbox Live and start downloading (thus encouraging more developers to make downloadable content, etc).
Xbox Live, again, is a big part of Microsoft's strategy to hook gamers. The Xbox Live account, Multi-player online, Live Arcade, Marketplace, downloadable demos and previews, Gold and Silver accounts, et al. 360 buyers won't even have to think about where all this stuff is going to be stored, all they need to do is decide whether they want a Xbox Live Gold or Silver account.
We already know that developers have expressed their opinions on the subject, and that there are several games using the 360 HD. Because it's included with every Xbox 360 there's no guessing how many people will buy an add-on AND the game. Every Xbox 360, at least at first, will have an HD. I can only reason that a game which needs (or would benefit from) an HD stands a greater chance of success on a console that comes with one than on a console that doesn't [come with a HD].
Xbox Live is a killer service, and the new additions are going to make it that much better. I hope developers make full use of it, and I hope that more consumers catch on.