Scenario: small company (<200 users), local Active Directory (single domain, 2008R2 level), on-premises deployment of Exchange 2010, Lync 2010 and SharePoint 2013.
The company wants to go full-cloud for Exchange, Lync and SharePoint services, thus is migrating everything to Office 365; however, the local AD will be preserved as the main authentication system, because there are various other application servers and keeping users and computers joined to the domain is a requirement.
In this scenario, should ADFS be used or will DirSync suffice?
Is ADFS required or optional?
If optional, what benefits (and caveats) will it provide over the simpler DirSync-only solution?