My tentative conclusion was that the giant difference between 1978 when the Lamport clocks paper was published, and 2012, is access to GPS. The fact that we get to take advantage of a DoD-built fleet of dozens of satellites that are constantly broadcasting extremely accurate times is pretty extravagant. Google can build systems today that rely on clocks in data-centers thousands of miles apart both being very close to "absolute" time, with explicit, conservative bounds on their error, in part through heavy use of GPS clocks. But for most of us that don't have an elaborate system of fancy clocks in our data-centers, Lamport's approach of having machines update one another's clocks (be they logical or physical) as they exchange messages is still a lot more relevant.
More fundamentally, though, Spanner doesn't involve absolute synchronicity in any real sense. Instead, at least as described in their paper, the focus is on using the special clocks to improve some system properties by bounding absolute clock skew. The ideas of causally related operations, and the orderings between these operations, is still important even in that environment.