Except in a few cases, which I’ll explain, the apps that appear in the multitasking bar are not currently running. When you press the home button, iOS will tell the app to quit. In almost all cases, it quits, it stops using CPU time (and hence battery) and the memory it was using is eventually recovered if required.
The thought that apps in the multi-tasking tray are running seems to be growing. I think Apple’s UI for this could be a little more clear.
That said, if people go through and “quit” all these apps every once in a while, is that a big deal?