That's a big myth. Exchange 2007 is great for a number of reasons. It expands the size of mailboxes so that some organizations can have bigger quotas on employees if they so desire. (Most don't because of the storage associated with that.) They also have some personal folder expansions where an employee can drag and drop messages from one folder into another and set retention periods on that folder. But, all of the folders in Exchange count against your quota. So, you really aren't saving or truly archiving messages, you're just setting retention periods – you're not moving them out of the primary environment.
Exchange 2007 does have some capabilities. For example, journaling by individual mailbox that allows archiving to happen much more smoothly and allow people to selectively archive. They also have transport rules to help with message classification. So I think Exchange 2007 as a stand-alone archiving solution isn't really a great choice. But having it as a part of an archiving environment, where it is feeding an archive software is a great alternative.
Go back to the beginning of the Email Archiving in 2008 FAQ Guide.