The first reason, of course, is to get experience on the actual hardware -- being able to load up uber-amounts of RAM and see the performance improvements first-hand. If that doesn't convince you, though, try this.
but if you run the Exchange 2007 beta 2 installers, you'll see that the Setup application links you to specific pre-requisite programs -- the .NET 2.0 framework, MMC 3.0, and the Monad (I mean "Powershell") bits. The MMC link takes you to a web page from which you can download both x86 or x64 versions separately, but the Monad link takes you directly to the downloadable executable (which looks to be a pre-release build of Monad). This is a minor problem for me; because of how often I find myself building out VMs, I like to get all of my pre-requisite software downloaded ahead of time and then combined into a custom ISO image I create. Since I'm not running a 64-bit version of Windows, I can't run the x64 installer, and thus can't get the URL for the 64-bit version of the Monad shell Exchange wants to use.
The truly interesting bit, though, is captured in the following screenshot:
If you click that link, it takes you here: the System Requirements page for the "Forefront Security for Exchange Server Beta" product, in essence the next generation of the Antigen product (the message hygiene technology acquired from Sybari). I got really excited for a minute until I noticed the text "Beta version not available" under the "32-bit Trial" heading. Le sigh.
Learn more about the Forefront line of products and services at the Forefront home page. I didn't see a separate announcement through other channels; I'm somewhat surprised.