One advantange would be the aeroshifters. Tri bikes don't climb as easily as road bikes & to have your shifting right there with just a flick of your hand makes a nice difference. On a hilly ride on my tri bike, I shift A LOT.
Plus, the radical "nose down" positioning on a tri frame apparently saves your hammies for the run.
I've only ridden tri frames, so I don't know if any of this is better than road frames, but b/c I exclusively do triathlons with a few rallies here & there, it works for me.