Please don't drink the Kool-Aid. It's like when Jobs used the battery as an excuse for not having 3G at first. There is a piece of the truth so he can't get sued, but not the whole truth. Never, ever take Jobs' words at face value. He's a salesman.
Yes, a highly active background app can chew up the battery. Examples incude Apple's own iPod player, and to a lesser extent, push email. Yet Apple doesn't ban those tasks... they simply explain that they are optional and use more power.
If you play music in the background, you expect your battery to run down faster. The same applies to anyone using Pandora in the background on other phones... but it's your choice.
And of course Apple has to keep the phone and SMS / MMS clients "running". Those are examples of most multitasking: apps waiting on user or network input, but not using any cpu cycles.
Again, that's true of most Palm apps... they're just on the deck, using no cycles, but waiting for you to bring them forward.
The number one reason Apple doesn't like multitasking is because they didn't put enough RAM in the phone. A close second reason is because they feared slowdowns in the UI. Stability worries would come next. Battery usage is pretty low on the reason list.