Isn't the purpose of tapping the top of the screen more to get you to the address bar so you can type another URL rather than a scrolling to the top function?
Also, most web pages have menus at the top of their web site but not as much on the bottom.
The tap on top (the phone's status bar typically) has been a scroll to the top feature way before Safari implemented hiding of the omnibar, and worked, and still works throughout the OS and other apps.
It's not really connected to Safari or its current (iOS 7) features, although now taping on top (just as tapping on the bottom, or simply scrolling back up a bit) brings the omnibar and the toolbar at the bottom, while a second tap will scroll to the top.
As far as quickly scrolling to the bottom, it can be useful in long emails, long lists, long pages, etc. Sure, purely from web page controls/options point of view, top is more important, but given that the scroll to top feature is there in general (outside of Safari and web pages), a similar scroll to the bottom one wouldn't be that crazy of an idea--although, yes, it probably wouldn't be used as often or by as many, and there would need to be some way to figure out how to implement it, given that there isn't really something like a phone status bar that could be used for those purposes.