Yup. If you have 5 (or more) weapons of the same type as the Phantom weapon, then they all gain a big bunch of base stats. Plus, you activate the skill of the weapon, which is pretty strong. Except for Guns, which is like, 45% HP. Because fuck Thunder.That's a good question. Since the Phantom weapons generally don't have Assault in them, you trade Assault for a big bunch of base stats. Secondly, those who aren't megawhales don't exactly get to choose what is in their Grids, so it doesn't seem all that impressive for most. Yet, people keep talking as if the Phantom Grids will be the second coming of God.
Meh, I guess I'll find out when I finally get my more advanced damage calc done which can calculate Phantom weapons as well.