You need it to take advantage of 4GB+ of memory really - so that's most systems you see in shops already. 64bit took off in Enterprise systems (IBM/Sun/SGI etc.) where it was common to have 4GB+ of memory years ago. The Desktop is next. 64bit will come to mobile/embedded when they have 4GB+ of memory.
The Windows and Mac OS ecosystems (particularly drivers) need to catch up with 64bit support, currently this is still spotty which is inhibiting adoption. Linux support driver support for 64bit is excellent due to most drivers being open source - but most people are not using Linux.
Yes I know. But if you are running a game on a PC with 8GB of memory and the game can only use 4GB of that (actually 2.7GB in windows), it's not really taking advantage of it, is it?
The Windows and Mac OS ecosystems (particularly drivers) need to catch up with 64bit support, currently this is still spotty which is inhibiting adoption. Linux support driver support for 64bit is excellent due to most drivers being open source - but most people are not using Linux.