I think you need to define "bad" trades. For too many traders a "bad" trade is a trade resulting in a loss. For me, the only bad trades are the ones made out of my rules, win or lose. I had my fair share of "Profitable" trades which, based on the wrong reasons for trading, were teaching me the wrong lessons. If you trade with probabilities, the x% losses that your system generates are not bad trades.
Now, by looking at bad trades all those trades out of the rules of the system, one can learn, ie, profits taken too soon, impatience in entry, not respecting a stop loss because, "this time is different", etc..