Thank you! I thought I had read this one through but I must have got lost somewhere 😛
Ah, I can see there are indeed restrictions... page 18-19:
License Mobility is a use right that is available for all editions of SQL Server 2012 software licenses with active Software Assurance (SA) coverage. With this SA benefit, customers can re-assign SQL Server licenses to different servers within a server farm as often as needed. Customers can also reassign licenses to third party shared servers. License Mobility is available for licenses under both the Per Core and Server+CAL license models.
? SQL Server licenses that are not covered with active SA can only be reassigned to a different server within a server farm once every 90 days, and they cannot be reassigned to a third party web hoster or non-private cloud at any time. (In the event of permanent hardware failure, the 90-day reassignment limit is waived.)
? All SQL Server licenses with active SA can be reassigned to another server within the server farm as often as needed; however, they can only be reassigned to another server in another server farm, or to a non-private cloud, once every 90 days.
A server farm may consist of up to two data centers located in time zones that are within four hours of one another and/or with the European Union (EU) and/or European Free Trade Association (EFTA).
A given data center may only be part of one server farm.
? License Mobility use rights do not apply to SQL Server 2012 PDW software.
So, without SA, you can only use your secondary for DR warm standby, and you can't really fail over to it to maintain availability of your systems while you are doing patching or Windows updates on the primary (which may/should be more frequent than once every 90 days)...
I have just checked the SQL 2014 licensing guide, and the wording is the same.