It's got nothing to do with prayers like piety. Actually, in this scenario, using piety is a good idea. The point is that when two people "safe" as in: use things that increase their defence, the fight will last longer, which means that you will use several ks in potions and food, and in the end everybody loses supplies. If two adversaries use no protective prayer, no defense potions, no food, no teleports, no running to safe spots, and only use offensive bonuses from prayers/potions, the fight will end very quickly, and both parties save money that they would have otherwise used up in supplies. The bottom line is, there is no need to make the fight last longer than it needs to be, because the longer it goes on, the more you waste money on supplies. Say you waste 20k in supplies per fight, at 20 fights, that's already 400k. Like he said, in the end, it's the people who sold you the supplies that are making the big bucks off your back.