I have never seen that option when buying a ticket. But its not those who HAVE paid, its those who bring boxes of Creamy donuts, Duty Free booze and such like on top of their 7kg bag.
In my experience, very few passengers access their cabin luggage during the flight, so why have the hassle. Unless it's the only luggage you have, check it in the hold, you'll have to wait for your main bag anyway.