It's all about Fascinate. The DC is based on your perform check, which is easy to get high into the stratosphere. Skill Focus, mw instrument, max ranks, high Cha, circlet of persuasion, cloak of cha, choker of eloquence. Admittedly, it doesn't work well on undead, constructs, and other similar creatures, but a few feats can fix that.
Completely core sample build: 6th level bard with 4th level bard cohort just for example.
(+6) 15 Cha base + 1 4th level boost + circlet of persuasion (only magic item)
+9 max ranks
+3 skill focus
+2 mw instrument
+2 inspire competence (from cohort)
Total = +22
Fascinate DC = d20 + 22
You find me a pair of creatures (since the bard could affect up to 2) you'd be fighting at 6th level that could come even close to making that saving throw reliably. And on top of that he can give them up to 5 suggestions with a DC of 16? The bard can grind every combat to a halt with a note from his lute. And it just gets worse at higher levels.