It really depends on the situation and the "spirit" of the defiance. I think I would find the encounter you described fun. I also love interrogation scenes where resistance is part of the plan. I honestly can't recall a session where a sub genuinely resisted - I am sure it has to have happened, but evidently, it wasn't memorable.

On another topic . . . your and My idea of topping from the bottom is very different. I don't consider begging to be topping from the bottom. In many ways, it's a sign that I have been effective in getting a sub to a weakened state. In the scene you describe, when you said "mercy", that was spontaneous and would have excited Me greatly. That being said, I would rather a sub err on the side of reticence to prevent topping from the bottom - which is truly a pet peeve of most Dommes.