I wouldnt say it would ruin them, but it may not do the electronics and favors. I had to remove the coil from one of mine and sand...get a slight covering of rust off of it so it would fire.
I usually keep 3-5 plastic bags like you get groceries in or from a conveinance store in the truck, and if I get caught out in a rain storm I put 1-2 over the enginge and tie them around the shaft till I get home.
I think trimmer trap and W E Chapps make a cover for trimmer engines also
To my dad, Thanx Pops you are my dad, my brother , and my best friend