Engine Inlet Covers when RON


This is just a question of curiosity more than anything. Thanks in advance for any input. What do you folks who fly the jets and for most part the turboprops use as a criteria for placing engine covers on when RON. Its been my observations less than 20-30% of operators place engine covers on their jets when remaining overnight or even for a few days. It seems they consistently cover other outlets religiously but rarely place engine covers over the engines. Is their some reason not to cover them? Is it more trouble than its worth. It would seem to be relatively cheap insurance against a potentially expensive repair. Again thanks for any input on my question.