Actually Barbosa is a 39% 3pt shooter. One year he hit 2 and a half a game at 43%. I'd say he's better than "about good enough not to leave alone."
My concern with this statement is that he's shot 32% and 33% the last two years. His first six years in the league he did shoot 3's much better, but his shot has been off. I'd say it was the Steve Nash effect, but the first of his bad shooting years was in Phoenix.
Barbosa had an injury that year in Phoenix. It effected his shot. I think it was his wrist but I am not 100% sure. He had problems with it all year long.
I think his poor shooting in Toronto is solely down to that team's inability to create quality shot attempts for it's players. They had nobody who could draw attention defensively and free up others for higher percentage shots with Chris Bosh gone.
Barbosa wasn't the only player on that Raptors roster to see his shooting sink. Calderon did too. Bargnani is another. Two very talented shooters. Kleiza is another shooter who suffered.
With Dwyane Wade there to draw help defense and able to find shooters for high percentage shots, I think Barbosa will be very effective from downtown once again.