Vent covers serve at least three main purposes:
1. Allow you to keep the RV ventilated with fresh air even while it's raining.
2. And just as important, IMHO, is to keep the sun's UV rays from constantly ruining/cracking the plastic material that most vents are made of. I replaced a few before I got wise and started protecting them with Maxxair vent covers, which seem to be made from a great material that is resistant to UV damage.
3. Allow you to have the vents open for ventilation in high winds, which otherwise might damage the vents or their operating mechanisms.
By the way .... how in the world could someone escape out of a 12-14 inch roof vent opening?? (I wouldn't want a roof opening larger than that due to the increased potential for cracking from the sun and leaking around the seal after a few years of exposure to the elements.)