Well, a standard Diplomacy check to go from Hostile to Unfriendly is only DC 20, or DC 30 as a full round action.
A level 7 bard should have about a 50% chance to hit a DC 30 Diplomacy check without any real investment beyond taking some ranks and having a good Charisma (figure 10 ranks +4 synergy +5 Charisma = +19).
So, this is remains a clever but not broken trick - out of combat you can periodically Wanderer's Diplomacy your way through any tough encounter. In combat, you can spend two actions to convert someone from Hostile to Helpful, three if you didn't pre-cast Glibness.