We may be able to draw some insight from the Apple Store app as well. Like iBooks it was shipped separately. Like the App Store and iTunes apps the Apple Store app lets you make purchases from Apple.
Is it simpler to have all the "purchasing apps" together, bundled with the iPhone/iPad or to strip away more system apps? The iPad didn't ship with calculator, stocks etc. The answer might go right back to Gruber's point: the iPad itself shipped out of the OS release cycle while the Apple Store app shipped just before the new OS in part to aid with the purchasing of iPhone4. Maybe both of these will be included in iOS5 next year.
