I really agree with this ... On both my iPad2 and iPhone4, both running stock iOS5.0.1, I've had problems with low memory leading to crashes! I keep the task bar clean a couple times a day and definitely for anything that updates in the background or uses GPS. I've also taken to closing out the tabs in Safari for the same reason (they can download in the background).
Originally Posted by SaberSerene
The OP may be right about closing apps not having an effect on battery life (both my iDevices have outstandingly good battery life, even on busy days). But the stability factor is a definite reason to keepthings "clean."