64 Bits are Overrated

"I think they are doing a marketing gimmick. There's zero benefit a consumer gets from that. Predominantly... you need it for memory addressability beyond 4GB. That's it. You don't really need it for performance, and the kinds of applications that 64-bit get used in mostly are large, server-class applications." -- Anand Chandrasekher

most desktop apps never use more than 1 gigabyte of memory (mostly due to the von-neumann bottleneck). And 32bit still allows using more than 4 gigs of memory:
http://en.wikipedia.org/wiki/Physical_Address_Extension

So basically 64 bits just waste transistors.

You don't really need it for performance

people think that 64 bit have better performance because AMD64 has better performance because than x86 because it had much more registers unlike the limited of the x86.

The best would be to have variable length word size just like some old IBMs but programming languages like C would not be able to use that. A modern lisp machine would hide things like word size from the programmer