What makes the spanking and exhibitionism appealing is that they both stem from Sam and Dean's intense love and desire for each other. Just plain spanking? Hey look at me being sexual? Kinda blah. But when it arises from "I love you so fucking much and want to you take me down to my most primal urges, want you to see me give you EVERYTHING, want to take something intense like pleasure/pain out of love and desire for you, want other people to SEE what you do to me, how crazy you make me, show them how beautiful and hot you are by what reactions you get from me"... well, that's just astonishingly hot. Almost any kink or form of play would be, with that at the center.