There IS a reason to have a Hancock Seraphim.
It's because, SHE IS BEAUTIFUL.
Seriously though, Did we get confirmation that none of them have DF's at all? I think in 1062, it was said that Kuma mini was a "problem child" whatever that means. We have to wait and see.
Also, Maybe Vegapunk had limited resources to create them. Like, i don't think any of them gave DNA willingly. So they probably secretly took it when they were in a WG facility like in a meeting. So he can't mass produce them like he can a Pacifista.
But otherwise yeah. It's like you said. There is more advantage in 7 Mihawk seraphim than any other combination if they have no DFs.