You can use it from Mission 17 and upward, including whenever you start a clear game you can use it from the start. You can only use it's DT for one battle. I don't think it's useless, it's by far the strongest weapon in DMC and the series in general. It has all the moves of Dante's DMC swords, and it's as powerful as a DT'ed Alastor. It's only disadvantage is not being able to DT outside of the Mundus battle. And that's mostly to make up for it's already broken strength, they had to balance the weapon somehow, so removing DT was the easiest. If you could use the Sparda DT in normal gameplay, there'd be no reason to use any other weapons.
It's DT is also the best in the game, for the short time you can use it.