Hi there,
the reason why airbus puts a clear minimum temperature in its environmental enveloppe is because of airframe flexibility as well a fuel freezing points.
Airframe flexibility means that the structure ( especially wings) becomes too rigid due to the extreme cold to absorb any possible turbulence encountered.
The reason why min temp on ground is usually about 20 degrees higher that min temp on altitude is simply because of the effect of TAT when flying. Usuallay TAT is 20 deg higher than SAT when flying at M.79.
So in case you encounter temperatures this low you will not fall out of the sky nor will you freeze to death, but you do have to descent to warmer atmospheres ( watch out for temp inversions though!
) just to put yourself in the safe zone in case of encountered turbulence.
A secondary reason are the fuel freezing points off course. Long periods of cold soak, especially in an A320, might cause fuel temp to drop below its min temperature. This could have an impact on several other systems as well on the viscosity of the fuel itself.
Hope this helps...