I just wanted to add this gem for consideration:
It's a good point. How can I argue with that logic?

4. 64-bitness: Leopard is the first 64-bit only version of a desktop client. Vista comes in 32-bit and 64-bit varieties. And most expect Windows Seven will still be available in 32-bit flavors. Until 32-bit machines go away, it seems like a good idea to offer 32-bit operating systems.
It's a good point. How can I argue with that logic?