I love Harrison Barnes, and he would be good value for Pierce, but I'm not sure that it's the best move for us. Green has shown he can be the starting SF going forward. I would rather use Pierce and whatever other assets needed to try to get an impact big man or lottery picks in a future draft.
Yeah if we are sticking with Green as our SF going forward Barnes is a bit redundant, but I would only be doing this deal if I was dealing Green in a separate trade to fill our front court holes (going after guys like Cousins or Gortat).
It wouldn't be a smart move for the Warriors either.
I actually think it makes a lot of sense for the Warriors. After so many years of losing they are finally relevant again and Pierce would give them a legitimate chance to get to the Finals. If you replace Pierce's production with Barnes's the Warriors are seriously dangerous.
Even at his age Pierce is a top 7 if not top 5 SF and Barnes is still just showing flashes of his potential. Its a major upgrade and provides the veteran leadership their team lacks.
It would have made more sense as a trade deadline move this season, but I think it could still work if the Warriors think it would be the difference in a championship run.