It will NOT cause overheating. I use my rMBP all the time and when it is not in use, I have it in a padded laptop case and carry it in my book bag. I'm in college and there have been numerous times where, even when being careful, I've hit it into desks, chairs, walls, you name it! The case has taken all the damage and left the macbook pro in excellent condition.
It also offers a layer of protection against accidental compression on the screen. The screen cover will take the brunt of this instead of the back of the macbook screen.
Also, the rubberized bottom portion keeps the computer from slipping around on the desk and makes it stay put while typing in your lap.
So many pros to getting a case. There is a megathread on this topic as well.