But the iPhone apps, I believe, are never closed. They are always running so I don't think your point is valid. That's just my thinking, I can be wrong though.Force quitting apps all the time will eventually create corrupt preference files. When an app closes normally (or in the case of the iPhone, the app is switched to the background), it would write info back to it's caches and preference files. If you force quit it every time, it would never have a chance to do this, and eventually the state of the caches/preferences will not match the actual state of the app and you will get crashes.
Doing this constantly is a recipe for an unstable phone.