What are the advantages of running a 64-bit kernel?
The way Mac OS X is designed to take advantage of more then 4GB memory pre 64-bit, causes it to swap address for memory above the 4GB barrier, which in turn makes each memory operation slightly slower. If you have more then 4GB of memory in your computer, running a 64-bit kernel will speed up memory read/write some, but will cause more compatibility issues with kexts. Thats it. If you have less then 4GB of memory a 64-bit kernel will have no advantage what so ever, and above that you will see some performance improvement when more then 4GB of memory is consistently being used. Obviously as no mac ships with more then 4GB of memory by default, and only more recent macs (excluding xserves and Mac Pros) can even accept more then 4GB of memory, and as it will take third parties some time to write 64-bit kexts, it makes absolutely 100% sense to default to a 32-bit kernel at this point.