A couple of thoughts. First, let's assume that Apple will do whatever is necessary to prevent the unpredictable lag that accompanies multiple apps running simultaneously. That means the multitasking features of iOS are severely restricted.
Second, let's assume that Apple wants to limit cpu load so that a relatively weak processor (designed for a phone rather than a pc) can be used in the iPad.
Third, Apple wants to limit the screen to a 1064x768 resolution for cost, performance, and battery drain reasons.
Finally, assume that Apple sees little benefit (and numerous issues for developers) in supporting multiple task windows on a device as small as the iPad's 9" screen.
Take all of these constraints into consideration and it's more understandable why Apple severely restricts the number and type of apps that can be run simultaneously. Music streaming apps, for example, require very little cpu load and virtually no screen real estate. Keep in mind that while you can listen to music while reading, you cannot control the music app without suspending the e-Reader app and even then only some functionality is provided for the music app. To change radio stations, for example, you have to exit the e-Reader.
A note taking app on the other hand, might be used alone or as you suggested in conjunction with an e-Reader app. In that case, task window resizing would have to be supported in some fashion. And while the number of music streaming apps (including Apple's own iPod app) is limited, there are numerous note taking apps (both typed and handwritten) that would have to be supported and made compatible with one another.
I, too, find some of the constraints Apple has imposed in terms of multitasking frustrating. On the other hand, I understand why they've made some of the choices they've made to contain the cost and weight of the iPad and to assure its consistent performance. As you suggest, technology advances (and the associated reduced costs) will almost undoubtedly allow for more flexibility and stronger performance in tablet devices. Apple is unlikely to be on the cutting edge of those developments, but they're likely to incorporate them gradually as they manage to meet their internal design objectives.
