User blog comment:TAGDavid/Tips/@comment-6881157-20140719033949/@comment-6881157-20140723112101

It depends on the deck the opponent is using and the situation (what cards are on the field, what resources each player has, etc.). If your opponent only has 1 crit monsters then yes, it is worth nullifying. If that one damage puts you in danger of losing to an impact then yes, it is worth nullifying.

Besides, if you attack with a 1 crit monster 2 turns in a row and your opponent doesn't nullify it, you managed to do 2 damage. If you attack with a 3 crit monster 2 turns in a row but one attack is nullified, you managed to do 3 damage AND your opponent lost a card in their hand.

The reason people let low crit attacks hit is that they don't really matter that much. Yes, it's better than not attacking at all, but that doesn't make it great.