One good practice for a DR is that any task should be able to be done by someone outside the team that normally looks after the service. This means the process for getting Exchange working should be planned and documented so that (say) a DBA can get Exchange working in a DR. Equally, is should be possible for (say) a SAN administrator to get SQL Server working. The whole point of an unplanned DR is that you should excpect reduced staff availability, either because they are a casualty or they cannot get to the DR site.

Personally, I would look on a DR process that recovers SQL Server by restoring system DBs as a higher risk solution than a process that keeps the system DBs at the DR site up to date. It may be a pain to apply configuration changes to a primary and a DR site, but when a real DR happens it should be far easier to get SQL Server working.