I'd think it'd be more because Pirohiko is the past Unlosing Ranger, so when he enters battle he'd be wearing the same thing the MC's normally wearing... So there wouldn't really be any difference?
But Piro's everyday attire is cool in itself... I agree that they should have made it into a costume... Maybe as reward for killing 500 or 1000 Silver Spikeyz... (It'd definitely be deserving of something awesome... I can only kill around 20 or so every run before getting close to death from starvation...)
But then again it'd clash with Pirohiko's entire story of only ever being in battle as the Unlosing Ranger... I'd guess that would mainly be why...