Well my argument is based on the mathematical average results to expect from the weapons. A powerfist will kill a praetor in the first round, even without a second specialist weapon or master crafted or having charged. A paragon blade or blade of perdition won't, even with charge, master craft and second weapon bonus. That's just the math.
Then you look at the point values. A paragon blade is 25, a blade of perdition is 20, and a powerfist is 20. It's the cheapest, most consistent and most versatile of the options. It also gets discounted in terminator armour, something thats auto take if running footslogging, dropping its cost to 5 points.
I'm beginning to see the outlines of the next project: a kitbashed HQ with Artificer Armor, a Power Fist, a combi-bolter, and a Jump Pack. Given that my DoR Praetors' body guard unit are 10 Dawnbreakers with the Champion wielding a BoP, the option to field a Praetor with a Fist would make the entire unit handle more enemy types. My Terminator Praetors are all armed with BoPs and Combi-Volkites but I run them with Terminator Command Squads with Chain Fists, so that line of thinking exists for my lists that use other RoW. I may do another Terminator Praetor with a Fist and a Blade for Rule of Cool at some point but got enough stuff to work on for now.