I can see your point, but surely having lots of apps running in the background is using some of the phones processing power?
A botton would be stupid, no offense. Isn't it already easy enough to close running apps by double tapping the home botton.
For the very rare occasions when you need to close them, yes - but the OP wanted to close ALL running apps, which would be a very annoying task with the current method. But, it's not necessary.
I have had apps crashed on him, by closing every app in the multi taskbar,I was able to reopen the app without the problems.
Bottomline, having apps in the multitasking tray does affecting the processing power.
OP What you are are looking for is the Cydia Tweak KillBackground.
One tap and all background/multitask apps are closed. ( Of course you need to be jailbroken to obtain this app)