If you never occasionally quit all your background apps, you end up with every single one you've opened in the app switcher. This can be a major pain when it comes time to actually force quit an app, or shuffle through the app switcher to select an app. Countless apps zinging by in the app switcher, cluttering up the works.
Although quiting apps may not save you any power, doing so will clean up the app switcher. So that's worth it to me.... Even if it uses another mA of battery power.