when do you really need a 64bit cpu?

when do you really need a 64bit cpu? So... I pose this question. When do you really need a 64bit CPU <other than the obvious potential need of a megasever needing the address space>? Most of the folks I know that have an Opteron or Athlon64 do not have any 64bit specific software, except maybe the Linux kernel compile. The Duron and Athlon32/Barton both seem to be replaced by the single Sempron32.