1) The feature licensing requires Enterprise CALs. Thankfully it does not require Exchange 2010 Enterprise. However, there is still a significant difference in cost between a Standard Exchange 2010 user CAL and an Enterprise Exchange 2010 user CAL.
2) The availability of this functionality to end-users is limited to Outlook Web Access, Outlook 2010, and Outlook 2007 (minimum SP2), the latter having only limited functionality.
3) Most businesses use third-party software for email archiving, allowing them to find a product that fits into their organization. Some simply set quotas and limit the total amount of data an end-user can have at all. Regardless, you wind up being charged for a feature you don’t use.