Silhouette was a holdover from World of Warcraft because World of Warcraft had very prominent (often in-world) PVP and having a burst-heavy class like Thief mimic a warrior tank and get ignored, only to delete his aggressors would be a rather serious issue. FFXIV, however, is very different in basic design from WoW. For one, every single job has a distinct (and quite prominent) weapon type and animations. I am in no way at risk of mistaking the DRK for a NIN because I can see very clearly that big honkin' greatsword he is using.
I don't think Yoshi-P is intending to be misleading here, but I am pretty sure the actual reason is that the game has already invested heavily into an economy of duplicate gear set designs that removing glamour restrictions would create a logistical nightmare with regards to that economy and potentially mogstation items as well.