"640K ought to be enough for anybody." -- Bill Gates
I think by now everybody knows that he never said that. This is one of the most over-used made-up quotes of all time.
That being said, probably without even knowing it, you pointed at THE reason why the 64-Bit architecture used in the iPhone 5S doesn't serve any real purpose at all: The phone has less than 4 GB RAM, so you simply don't need a 64-Bit architecture. At this point, the processor is busy with shoveling a lot of unused zeros around and the performance gain has nothing to do with the 64-Bit design of the processor; that all comes down to the other optimizations in the CPU and its supporting hardware.
The only reason they put a 64-Bit processor in their current machines is to lay a foundation for future use so that software developers might have an incentive to begin porting their software. But by the time 64-Bit hardware becomes the standard, or at least useful in a mobile gadget, your current hardware will be entirely obsolete and long forgotten --> that's the typical fate of any first generation hardware.
ARM have released the first 64-Bit designs a couple of years ago, but we are still at least three to four years away from the time when 64-Bit processors and 64-Bit software in mobile devices will turn into a critical mass.
Just look at the Mac. Apple switched to pure 64-Bit Intel processors in late 2006/early 2007 and most of the software for OS X is STILL pure 32-Bit software.
Microsoft made the 64-Bit design a first class citizen in Vista (although there were 64-Bit versions of Windows XP, too), but even until today most software for Windows still is pure 32-Bit.
Why? Because only applications like Photoshop or video editing software and maybe DAW solutions truly benefit from 64-Bit designs. For everything else, a 32-Bit address space will always be more than enough.
There are only few application domains where a 64-Bit architecture is what you need.
