There's been a lot of talk about "using [Belinelli] properly" over the course of the season. At this point I've heard just about every way we saw him being used accused of not being the "proper" way. So rather than just talk about it - here's a good compilation of his baskets in one season with the Spurs.
I think what we find is: hardly any shots curling off screens in the elbow/free throw line area. We do see him attacking the basket and creating offense (a lot of such plays aren't shown in a video comprising mostly him scoring - but he was very much a facilitator as well), so any argument about the Spurs not having him handle the ball is not true. We also see that (as one naturally would in a highlight video) he was knocking down a lot the open 3s that he was simply missing this season on very similar plays. We got a taste of him moving off the ball and cutting to the basket this season, but those plays require quick and crisp passes from bigs in the high post that we don't get from WCS or Koufos (or Cousins this season if memory serves me correctly. Most of the time Cousins would put the ball down and drive to the rim). But as far as give and gos on dribble hand-offs go, we did see some of that particularly with Koufos. There are a number of plays where the Spurs ran a pin down for Marco to get corner or wing 3s, and we got a taste of that as well in some late game situations and plays out of timeouts.
My conclusion: Based on the Spurs' use of Marco alone and holding that as the "proper" way to use him (questionable as it excludes summer international play and such) - the main area in which Karl "misused" Marco was having him come off screens for quick midrange pullups. Having him handle the ball and create offense, while at times excessive was not completely out of the ordinary or unthinkable. In addition, we need to factor in not only the plays and offense that Karl ran but the personnel on the team and their ability to allow Marco to have the kind of space or plays off of movement that we witnessed on the Spurs. WCS and Koufos are not Boris Diaw and Tim Duncan (not only in them passing, but think of all the baskets around the rim that KK blew off PnRs). Lastly, we have to accept that Belinelli himself simply shot poorly all season, even on open shots from 3.
The point of all this is to manage fans' expectations for next season if Belinelli is still on the team and the roster/rotation remains largely intact. While I certainly hope for an improvement under a new coach I don't think it's fair to say that George Karl alone was responsible or even chiefly responsible for a bad season-long performance by an NBA veteran. In my personal opinion which I hope I have explained in this post, Marco was not as "misused" as some would have others believe.
If you look at his career numbers, this is by far his least efficient and highest usage season. I think Marco has proven that he is a specialist. He is a perfect 7th man spot-shooter. The problem is that with Seth's height and Marco's poor defense, you cannot play them at the same time. Both are excellent 3-shooters (Marco has the career track-record, Seth has the recent evidence) but neither are good enough defenders at the 2 to start for a playoff team. Therein lies the issue. Looking around, Seth is essentially 6'2 and Marco is 6'5. If one of them were to have the raw measurables to play the 2 it would be Marco. Can he be taught defense though? 
