I don't think Bargnani will ever be as good as Dirk but no reasonable GM or any other high-up person in management would ever follow your line of thinking there.
Dirk is considered by some to be the best player in the NBA... and considered by all to be top 5. The chances of this guy being a top 20 player are next to none. But considering how often 7' white soft euros with jumpshots turn into NBA superstars are "worth it," I'd say don't pick him.
You have to have supreme faith in a player to take him over a guy who solves nearly every need your team has, is a better player now, and arguably has more potential. All the while this player is just like your draft picks of the last two years: 6'11, thin as a rail, jumpshooter, questionable in almost every area. Meanwhile you can get Aldridge. Lamarcus fricking Aldridge. And you turn that down for a one in a million shot at the next Dirk when you already have the next Dirk on your roster: Chris Bosh.
I just don't get it. HOW is this a decision. Unless Colangelo has a time machine and this guy is the next Larry Bird and KG rolled up into one, I can't see how anybody would consider taking Bart.