Q: Whirring down iPad apps.

Conserving power and, in rare cases, resident memory.

doesn't iOS put those apps you are not using into a 'frozen' status and thus has little impact on the devices power consumption?

Of course any App that supports background processing will potentially add to any energy usage when they run.

I seem to recall that iOS manages the apps you have open and whilst closing them means you do not clutter up the menu when doubling pressing the home button, the operating system looked after them without really any impact on your use of the device.

"These apps are not necessarily actively in use, open, or taking up system resources." as per http://support.apple.com/kb/HT4211?viewlocale=en_US&locale=en_US

Yes, it is true that many, perhaps the majority, of apps are actually not using system resources when the screen is. It activity engages, but there are few that do. What the impact is on system resources is hard to say, and it may be minimal.

When I answered the OP's question, I perhaps should have qualified it a bit.