It's easy to imagine wanting to use an iPad to read text in Mobile Safari, copy some text to a Pages document, and send that document to a colleague via Mail. That specific example may turn out to be possible with the current iPhone OS, but it points toward needing more ways for iPad apps to work together in the future.
Let me break down the user's workflow here:
- User reads text in Mobile Safari.
- User copies text to a Pages document.
- User e-mails that document to a colleague.
The flow of tasks is linear. The user does not need Mail open while reading the text in Safari, nor Safari open while pasting text in Pages. Whether a platform supports multiple simultaneous applications or not, users typically work with one at a time.
The advantage of multiple open apps is that the user can switch tasks really quickly (the other oft-quoted benefit, of being able to see context in multiple places at the same time, is actually a feature of a windowing UI: a different technology, and one that iPhone OS lacks). The disadvantage is a technical one—the operating system must allocate resources to applications that the user isn't currently working with. The iPhone (and, I presume, the iPad) provides fast task-switching anyway, through its recommendation that app developers retain app state on termination and recover it on launch. The act of moving between apps via the home screen is supposed to feel like switching tasks, even if it's implemented by a kind of pause-and-resume.