Bolt (and any other card that deals with Narset once it hits play on a 1 for 1 basis) is a really bad answer as your opponent has already fired off an Impulse. It's basic card advantage - you are down a card and your opponent is even with likely a restricted card replacing Narset.
Sometimes you just have to deal with a board state though which means a bad answer is better than no answer. Since there is truly no answer to narset that is not card disadvantage, you might as well use the most efficient answer you have available. Assuming narset resolves and your opponent immediately -2 her, would you rather spend a card to stop another -2 next turn or let him gain even more card advantage from it?