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!

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car