I know where you're coming from with low freq. impedance and low moving mass, but it's not really protecting the tweeter from damage or the amplifier
if the proper crossover attenuation and frequency are used.

I guess if you feel like you're uncertain of your crossover selection, the added attenuation would be useful.
I've never used a ribbon in a car. Although I've used capacitors in design and heard other ribbon systems utilizing capacitors to help with response problems, which have produced definite audible differences.
You are getting sag from the tweeter without a capacitor, npdang?