I think the worst thing about NPD traits is the desire to see oneself as almost perfect... like others deserve criticism for their "flaws" but not us..
And their is the propensity for us to have blind spots about our flaws and true abilities.

But, again, the benefits are that we keep going on... continuing to believe in ourselves. I think that desire could lead us to do good or great things... as long as we can temper it with a little objectivity.