That "need" is a bit of strong word, don't you think? You don't need a Phantom Hammer to run a Ascension Grid.
And putting a shitty weapon into a shitty Grid for a Phantom effect isn't going to do much, either. You're gaining 30% Assault and 30% HP (plus base stats on those Hammers) for the cost of 26% stats. Yeah, sure, it's an improvement. But it's not exactly anything groundbreaking, especially considering the complete waste of materials going into that Hammer.