On one hand, it would add some excellent customization options to the mostly same-looking high-tier weapons, the Vanguard, Faction, and Crucible ones specifically. On the other hand, it'd just be one more thing to add to the loot RNG to futher frustrate people. I say only have them available for purchase at Eva Levante's shop and/or The Speaker. Of course there's another issue, where people might complain about not knowing what kind of weapon a person has equipped until they get killed by it. To that, I say that that pretty much only matters in the Iron Banner. It'd be easy to disable shaders in that specific game type if it ever manages to become a significant problem.