We actually do exactly this already 
The key phrase for iOS is “Significant Location Change”, which is activated when switching cell towers, and not by GPS.
As a result, this feature does not drain your battery like GPS would. We regularly run power profiling as part of development, because we care a lot about battery drain too.
tl;dr We’re doing the same thing as Dark Sky 
Perhaps we should add this to our FAQ? Thanks for the feedback @keirwilliams