I spank bare bottom because there's a thought process to it. When I tell my wife she's going to get a spanking, and it's time, then she's the one who pulls everything down and bares her bottom. To me that's her consent. It's her way, even though she may be protesting verbally, of saying I'm going to submit to your will and get spanked of my own free will.
Sometimes after baring her bottom she plops herself across my lap as a final protest but it doesn't keep her from her spanking. I also spank bare bottom because I can see what is happening. In other words I can tell if I'm spanking too hard or too light. Once I see her bottom turn a nice red I can tell she's been taught her lesson.