Disembowel was nerfed so that MCH and BRD aren't so reliant on having a DRG in the party. If you do not bring a DRG, then that also sort of precludes bringing a MCH or BRD, because not having a DRG limits their DPS potential so severely.

These changes are not made in a vacuum. They don't just nerf an ability for such a reason without also considering the impact it would have on the original job. DRG received numerous other buffs and new actions to enhance their damage. They may have lost 5% damage from Disembowel, but if you look at DRG parse records, you'll also see that Mirage Dive accounts for about 5% of their DPS. Sadly the changes they made just weren't enough to bring DRG up to a level that is competitive with other melee dps. The excuse that DRG damage is lower because it brings more party dps is a flimsy one, because Battle Litany and Dragon Sight are so minor. The only real party benefit they are offering is the piercing debuff, but even if you do bring a MCH/BRD, it still doesn't really equalize DRG with other melee dps.