I think he can be effective in the Spurs' system. Parker doesn't really run the offense so much as the offense feeds off his aggression. A lot of times, Duncan or even Green would initiate plays, and Parker would just wait to get the ball for a mid-range. Why can't Joseph learn to do that? He's very fast and quick, and he's a good enough shooter to develop a mid-range game. I think the team should commit to him being the full-time backup and prioritizing him learning to play Parker's role in the offense. If anything, letting Ginobili dominate the ball for the second unit has hurt his growth.
That's different than it would be in the summer league, where most of the players would not know the system (by lack of experience and by design). Cory would have to get others involved and get his own shot more often. He's shown the tools to do that, although it would be interesting to see how he handles kick-outs when players aren't always in the best position to get the pass like the actual Spurs players tend to be.