Yeah, humidity (or the lack of) and air flow are key. Stuff can go wrong, probably more likely with newer cars and more sensitive electronics and such. The T/A spent 4-5 winters outside, covered, and I never noticed any real issues as a result. Last year and this year I did manage to get it indoors. The DTS sits outside all the time, and I may drive it a couple times a month...usually a flurry of use and then nothing for a couple weeks. I cover it also (assuming it's clean) and put in the battery maintainer every few weeks, it's been good.
I also have the SLK in my driveway right now, covered. I am sure it'll be fine. The interior was damp (from a previous leak, plus my cleaning efforts) so I ran a dehumidifier in it for a couple days and then put one of those 'damprid' containers in it to soak up excess moisture. Since it's here at home I'll probably start it every 6 weeks or so and let it get to temp, chase some of the moisture outta the engine bay.