The nice thing about her ultimate is that it has no mana cost, like the rest of her spells. Early on the damage is low, sure, but so is the rest of her damage. Once you've used her Dagger Throw and Shunpo there's really not much you can do till those two cooldown - her attack damage early without items is pretty ****. So you place yourself between the enemy and his tower and use your ultimate. If he stands and fights he'll take a lot of damage, if he disables you or tries to run, use Shunpo as soon as its CD ends to finish him off.