A Server and Cloud Enrolment requires commitment across your installed base of a particular product – SQL Server for example. If you haven’t got SA to renew, then you need to buy licences with Software Assurance for all of your servers – which can, of course, be expensive.
An alternative is to buy Subscription licences. But, do you get the same SA benefits for those Subscription licences as you do for the L&SA purchases? You do, and there’s new confirmation in the November 2015 Product Terms document – see page 31 for example.