It never had a cape when the game was launch, who said it needs a cape? I am going to assume you weren't on the forums when the change happened in patch 1.6.2 which caused http://www.swtor.com/community/showthread.php?t=583086
This is about fixes, not alterations that would cause an outbreak of people using the phantom/spymaster as a change to demand it back. You really want to slap a cape on this set so it clutters through mounts? Thats the last thing we should want.
You want a [Darth Revan's Chestpeice] with a cape. Suggest it in the cartel market threads for it to be purchasable from the vendor in crew skills.