At least in this game he does something!
Yes, that's right, I dare challenge that Diablo 3 Tyreal is more productive than Diablo 2. In Diablo 2 he let Baal get away, provided you with advice as you fought your way to (let's just say) the center of Hell where Diablo was at, and then he doesn't even help with stopping Baal, you have to do it. Imagine how much quicker you could have mowed down demons if you had the Angel of Justice fighting alongside you. Then at the end of LoD, he destroys the worldstone.
If you remove the cut scenes between acts, he only has one bit part in the smallest act of the entire game. In Diablo 3, he's fighting alongside you for chunks of Act 3, and Act 4. He's helping investigate the corruption of Caldeum in Act 2. Act 1, he's useless, but hasn't regained any of his knowledge until the end.
I'm guessing had they left Tyreal an Angel, he would have showed up in Act 4 and said "Hey, thanks for coming to help with Diablo invading my home. It's about time someone did something useful."