I thought I'd post a link to this nice article from jkOnTheRun:
Hosted Exchange 101. It's a great high-level summary of why someone might want Hosted Exchange. Here's a quick pull-quote:
What does Hosted Exchange provide?
Obviously, you have a centralized e-mail server with a hosted
Exchange plan, the same as you have with Gmail, Yahoo! Mail and the
like. There are a few key differences however; some may help you decide
if you need a hosted Exchange plan or not.
One key feature is that Exchange provides not just your mail and all
of your e-mail folders, but your Calendar appointments, Contacts, Notes
and Tasks. With one single client connected to Exchange, you can keep
all of these artifacts in sync. To this point in time, the ability to
have all of this information stored, integrated and backed up in one
central location has been one of the two main reasons I've kept a
hosted Exchange account.
The second reason is the support for Direct Push synchronization.
This is the ability to have all of my information on the Exchange
server sent to my many mobile devices in near real-time. I don't have
to worry about a manual send and receive function, nor do I need to
synchronize my clients with the Exchange server: it simply stays in
sync.
Nice!