Master trials should offer unique rewards you won't find anywhere else
I'm actually kind of wondering why SE is making more Master Trials without any real reward. This all sounds like a huge setup to release a new quest/content that requires either the base weapons, or the fights to be completed. They've quietly done things like this before, where they slowly added
seemingly useless quests and fights, only to turn on the "GOTCHA" button and make something amazing come from the entire questline (Mog Gardens, Imprimatur Coalitions, SoA Earring/Neck questline come to mind.).
I would not be surprised if these Master Trials and the cosmetic weapons are nothing more than Pre-requisites to something bigger they are planning. If history has proven anything, everyone should make an effort to clear Master Trials now so they aren't on the outside looking in if/when SE releases a
real reward from these fights.