Although I would also appreciate heated mirrors, I haven't found it to be the massive problem others have described. In freezing rain conditions they don't tend to ice up if you're moving - that's what happens to the front of the mirrors. I once drove for four hours on the 401 in the midst of a ridiculous ice storm, and ended up with ice an inch thick across the front of the car, including the mirrors. But the mirrors themselves never iced up. (No I am not exaggerating about the inch thick part - everyone at work the next day was asking me what the heck happened to my car. It looked really bizarre.)
Also, I don't change lanes without shoulder-checking, regardless of how good the mirrors are.
Finally, I should point out that you can turn on the rear view camera while driving - and it's a nice big screen on the Model S.