Most users would never see any effect, but if you build complex scenes it would be an asset. 

Geometry or texture-heavy scenes, complicated animations, running multiple dynamic cloth simulations, and 100+ particle effects could make use of a 64bit Poser executable.

~  edit:  cross-posted with bagginsbill  ~

Bagginsbill, do you mean that the Poser Pro 2010 executable is 64bit?  I have the beta installed on two 64bit machines, but I installed outside of the usual program files folder in order to avoid Vista's UAC antics, so I would not be able to distinguish.  I have pulled about eight and a half gigs total while rendering in separate process, but that says nothing of the executable.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5