there is a misconception about the need to enable the Outlook Anywhere in the Exchange 2010 servers as a pre-requisite to move mailboxes to the Exchange 2013 servers. Actually, if users don't need to access resources in the Exchange 2010 servers through OA you do not need to enable it. Sometimes, you can face a customer that has a security statement against the OA. In such scenario, you do not need to enable Outlook Anywhere in the Exchange 2010 servers only to migrate to the Exchange 2013 servers. Obviously OA must be always enabled in the Exchange 2013 servers "just" because this is the only way Outlook can connect to Exchange 2013.