An excerpt from an extensive Walt Mossberg review. I believe it would be preferable to make the multitasking mode a user option; some users would probably prefer true multitasking to optimized battery life.
After years of complaints, Apple finally has brought multitasking to the iPhone. But it has done so in a limited way that won't please everyone. On the iPhone 4, multitasking doesn't mean every app can work fully in the background. To prevent a disastrous drain on battery life, Apple has allowed only certain apps to fully multitask. These include streaming audio services like Pandora, which keep playing music from the Web while you do other things, and voice-prompted navigation apps, which keep working while you're on a call. Others that fully work in the background include Internet calling apps, and those that perform long downloads.
But some logical candidates, such as Twitter and Facebook, merely pause in place when you switch away from them. You can get back to them quickly, and they update more rapidly than before, but they don't constantly update in the background. They only wake up in the background if you have set them to notify you of an update, and then only for a limited time. Apple says constant fetching of hundreds of social-networking updates in the background would kill the battery too quickly.