Joseph is surely a smart players who knows how to play the game, has good size, and seems to be pretty good at everything.
However, let's keep things in perspective.
First and foremost, this has been four summer league games. The likes of Joe Forte, Kedrick Brown, J.R. Giddens, and Jeromoe Moiso all had their share of summer league success. It doesn't mean it translates to the NBA game.
Second, even if Joseph is an exception, the question I have is whether his skill set translates into being a role player on a team with a lot of deep talent. While he may have similar agility and body control to Pierce, as a role player lucky to see 5 minutes of action, he'll likely never have a play called for him where he can show it off.
The more important question will be whether he can do something like lock down other perimeter defenders, make the smart pass, and most importantly, knock down the open weakside jump shot.
Maybe he can do it. But I wouldn't bank on summer league success always translating into NBA success, particularly when most of these summer league guys are going to be asked to NOT be the man during the regular season.