Namazu greatly increase the survivability of DPSs at the beginning of the fight. In longer PvE fights where you struggle to keep DPSs alive, this soul would be a better choice than Odokuro. If your team is more established, Odokuro should have boarder use.
*2 set bonus only effective in PvE fights
Although the Unique Passives are not functional in PvP, the 1 set bonus does offer greater flexibility for soul composition.