That is definitely what the extra bulb is for. I have seen the same lighting system on BMWs and Lexus as well.
European drivers know about this system, but here in Texas, I get drivers pointing out that I have a tailight bulb out!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car