Not a big fan of these kinds of tweaks. I think iOS does a good job of freeing up memory when needed and free memory is wasted memory anyway. If you really want to get rid of open apps just respring your phone. Quick and easy.
It's more of a neat-freak thing for me...I just don't like seeing so many app icons in the switcher. But it seems to come in handy on occasion for memory reasons if I'm running a lot of intensive stuff at once. Mostly though I just let iOS handle it like you're suggesting, but I like having the option.