He has Chidori because It might not be fast enought to hit him. Rather if his shield does manage to be quick enough to go through block Sasuke's strike, then, it'll tear Gaara apart, cutting throught all his defences.

But then again, some might argue that the kunai might not be able to penetrate through Gaara's main sand skin. ... How about a kunai with Explosive tags on it? Or they'll have to throw it very very hard and very fast.