@agracier ...
I have never met a child, tween, teen, or adult who's level of sophistication (ie: sneakiness) doesn't increase in order to engage in prohibited behaviors, and avoid any resulting consequences (not just spankings).
Good traditional/proper spankings, when combined with other discipline for bad behavior, and rewards for good behavior, are an extremely effective way to help achieve desirable behavior modification.
Properly administered corporal punishment does not teach that "might makes right" any more so than other properly administered punishments of every kind. By definition, punishments are administered to those under authority, by those in authority. Obviously, it is the responsibility of an authority to lovingly and not-abusively administer discipline, in such ways as to achieve desirable behavior modification... for the benefit of the recipient of the punishment. It is short-sighted to say that good disciplinarians should not maintain discipline (including the judicial use of corporal punishment) because somebody somewhere will administer discipline in ineffective or abusive ways.
Regarding the straightforward question, why spank on a bare bottom? ...
Clearly, it's the obvious way to eliminate a large number of variables. For example, imagine a boy and a girl spanked for cheating on a math test...
The boy is spanked over his cotton school trousers and a pair of white cotton briefs. The girl is spanked over a thick pleated wool school skirt and three pair of panties, pulled on one over the top of another (a trick used by girls and boys when they know they're gonna be spanked).
Solution... trousers and underpants down... skirt up and panties down!
Also, a bared bottom can be monitored during a strapping/paddling/caning. Corporal punishment should be at least a little bit uncomfortable... and, if necessary, cause some discomfort for the recipient when sitting down for 24-48 hours, but not abusive! Warm scarlet buttocks, with perhaps a few stripes from a hickory switch, can be a great way to correct disobedience, but the severity of the redness and stripes needs to be commensurate with the severity of the disobedience that is being corrected. Therefore, good disciplinarians need to keep an eye on bare bottoms that are being chastised.
Lastly, required nudity can be an effective way to emphasize humble submission to disciplinary authority. As a boy, I knew that severe disobedience would result in me being required to strip completely naked and present my bare bottom for a whipping with my father's razor strop. For the most part, this pain/embarrassment motivated me to avoid severe disobedience. On occasion, the temptation to disobey was intense enough for me to try being sneaky, rather than being obedient to avoid the strop. This only happened a few times, because when I was busted for being extra-sneaky, I was rewarded with an extra trip to my father's workshop. Receiving his razor strop in the evening and again the following morning made it extra-risky to be extra-sneaky!
I remember my dad saying, "Son, your grandpa dealt with me severely when I engaged in premeditated deception... 40 in the evening and 40 in the morning... it worked well for me, and I suspect not sitting comfortably for a week will work well for you also!"
Dad: "Bend over the saw horse, son."
Me: "Yes, Sir "