In formal situations and formal writing. People use it usually at workplace when talking about their higher-ups - you'll hear they say "회장님께서...", "사장님께서...", "부장님께서...". If you say "고객님께서는" instead of "고객님은", you'll be a top salesman. But when it's spoken in daily conversation, it sounds kinda formal and "educated", so it's hardly used between friends. I heard someone got tackled and shunned because she used it with her friends who were turning into her haters - they considered her "posh". That's why I see it as a kind of an upper-class marker in spoken Korean.

I guess the reason you hardly heard of it is it's too honorific subject market... Like .. use this for grandparents to grandson and daughter.. for CEO or chairman in company.. etc... and also you can use '께서' for your important business customer... ex) Mat님께서는 이 상품에 대해서 어떻게 생각하세요? (what do you think about this product)