While some applications could benefit from 64bit even if it isn't using more then 4gb, most simply will not or not enough to warrant the extra development cost/time. Plus 64bit apps are not backward compatible so anyone stuck on a 32bit chip cant use it.
Difference between Leopard and Snow Leopard are more then just a simple evolution to 64bit, they cut down overall footprint of the OS and optimized parts of the OS. Kinda like windows 7 was a optimized vista. 64bit still is more suited for apps that need to use more then 4gb, your mostly wasting your time/money as a dev ATM adopting 64bit if you don't need it.
And what makes you think that they haven't optimised iOS 7 for the 64-bit processor? They've rewritten the stock apps to support it, so that's something that we know of -- and Apple doesn't really talk about that kind of depth much so no one knows what else they've done.