In previous versions of Exchange 2003-2007 there was a feature called SIS (Single Instance Storage). This meant if an email was sent to multiple mailboxes in a distribution group, it would only get stored in a mailbox database once. When designing database layout it was recommended you group users of a similar operational role under the same mailbox database. This ensures when emails are sent to distribution groups, the email is only stored in the database once.
In Exchange 2010, SIS has now been removed to help improve performance allowing low cost disk to be utilized. This was one of the key factors in reducing disk I/O on the Exchange database.
The decision behind this was around using low cost TIER2 SATA disk, who cares if emails are stored multiple times inside a database?
Have a read of Ross Smith IV's article explaining this in more detail: