If we already own the weapon, I can think of no single reason to get that exotic weapon again. The exotic piece comes in similar light but usually slightly lower than we are. So there's not even the benefit of getting a higher level weapon. So yeah, when the hell are we going to get armors only from Xur once we unlocked that exotic weapon? Why the heck would I want a second exotic weapon that I can pick up from my collection? This isn't even truly about being RNG anymore, it's literally just there to screw you out of 97 shards.