Interesting question. 64 bit is unnecessary and probably overkill for today's Android and Angry Birds. It is necessary if one wants to host a professional class OS either as a host-OS or virtual machine. There is no question that an x86-based smartphone will be made that will run Windows 8.1 or touch enabled Linux either in the foreground or background.
What would that mean to smartphone users if they could carry their entire desktop and apps with them and connect to docking stations that have keyboard and high resolution monitors?
You're right about this. ARM64 doesn't have any software to bring to the table, and if it's not above 4GB of ram what's the point? But when it's X86 there is a whole new world of possibilities if they can run legacy Windows software.