Average damage depends greatly on whether a party of that level has a bard or not. Inspire Courage is EXTREMELY powerful. Even with only one of the myriad boosting effects out there (such as the swift spell Inspirational Boost), that's +4 to hit and damage at 16th level. It's especially good for archers and for summoned allies, who are often two of a party's best weapons against a dragon.
16th-level ranger vs. AC 33, with Dragon as a +2 favored enemy, +6 Dex, +3 bow of +3 Str, WF, Haste, and Improved Rapid Shot: that's +27/+27/+27/+22/+17/+12 for 1d8+8 damage (avg 13.75 per hit without Improved Crit). Average damage is 42 pts per round.
Now give him +4 to hit and damage. Average damage shoots up to 76 pts per round, nearly double. Inspire Courage will be even better for PCs, animal companions, and summoned allies who have a lesser chance to hit than the hypothetical ranger does (the lower your attack bonus, the more that +4 helps you). Your basic Huge Air Elementals will go from 9.6 to 19.6 pts of damage per round (thanks to 57% more hits and 31% more damage per hit).
Give the dragon a higher AC (say, 36), and the relative benefit of Inspire Courage is even greater.