I believe it's an issue of CPU and power consumption control. Might be nice to have full multitasking but I haven't run into any application on my son's iPhone that I want running in the background that isn't one of those WP7 native applications. Of course I don't use a smartphone continuously through the day - it's possible I just haven't found that uber-app I want running at all times.
That's the same with the iPhone, always has been. Apple just didn't let third-party apps freely multitask like Android does (there is controlled multitasking now). WP7 is starting in Apple's initial position, no third-party multitasking. A good example of useful multitasking would be playing music and running your GPS navigation at the same time, and then being able to check your Facebook/WWW/etc without stopping the others.