Tonneau cover, or not

[quartetchiang]

So Porsche says to keep the tonneau cover on at all times while driving top-down. But, really? Aesthetics aside, must this instruction be religiously obeyed? Any sound mechanical argument to support it?

[GeorgeK]

Cover prevents dust accumulation that will cause abrasive damage over time.
Wind buffetting above a certain speed (about 50 mph in my car) will move the folded roof visibly enough that rubbing damage could occur as well.