Wait constitute as “valid”? Something that can actually be debated. Something that is objective and not feeling based such as “I don’t like this, I can’t see their real gear”. It isn’t a valid argument because why should your own feelings be taken into consideration over another player who does want transmog or who does want to see people express their character through its appearance.
This is why I offered the PvP one as an example. While I do not personally see it as valid or agree with it. It is still a valid argument against the early introduction of the transmog system if Blizzard decided they wanted to go that route.