Seems kind of pointless to use one unless you need it for a purpose. I wouldn't want to create a new point of possible noise induction or signal loss at a point unless it's necessary.
A lot of times I see people crossing their ribbons low, so they use it as you're saying in a complex crossover with multiple attenuation slopes. For example if the pass band of the ribbon isn't attenuated enough to compensate for the Fs, and a bump in response is present, you might want to use a cap to flatten that peak.
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?