Barrel distortion will exist in every lens (except perspective control lens and large format) , it is a matter of the degree of distortion. In the case of the 24-70 f2.8, I actually find it rather well control and not terrible as you have put it.
Back to the topic.
I find that I still prefer the 17-55mm on my D200 rather than the 24-70. The 24 - 70 on the D200 with the 1.5x crop is not wide enough on a number of occasions, where I have to shoot in a smaller area.
While the same is not for the case of the 24-70 on the D3, it is as wide as the 17-55mm.
Hence there is some truth in the sales person statement. Though you still can use, it is just the suitability.