I don't know how accurate it is to say it's silly people only play one class if they have a golden portrait. I mostly play Shaman because it was the class I felt I had the strongest cards in from when I started, but I play everything except Warrior and Paladin mainly, as I'm missing the class legendaries for those two.
Still, I have 300+ ranked wins on my Shaman, and I only play at most a couple of ranked games a day. It's inevitable that in another 6-12 months (or a little longer), basically everyone will have golden portraits without even trying for them, because even I will have them and I don't play all that much.
People stick to a couple classes because they tend to have the best set of epic/legendaries for a couple classes I think, and the golden portraits just come on their own naturally.