Talking this through last night, we decided here, that it was because we had a Rayburn going all the time, to heat the house and water, and naturally, it made semse to use the top plate as much as possible!
This was in the fifties though, and the new double saucepan was looked on rather like an electric bread maker would be today!
I wish I'd kept it, but there again, I don't miss the old Rayburn that much - it was a bit too wheezy for us...