One of the first things many users notice about Windows XP and 2003 x64 Editions is that they do not support any kind of shell integration (Deskbars, Toolbars, Context-menu handlers like WinRAR, etc). The reason for this is that 64-bit processes cannot communicate with 32-bit processes or libraries.
Running the 32-bit shell maintains all the benefits of having a 64-bit OS underneath, allows the full use of standard 32-bit Windows shell integration, while still allowing you to run both 32-bit and 64-bit applications side-by-side.