If you could do what he can do when you were 12, the natural follow-up question is why you are not the first team left back of Manchester City by now?
Jokes aside, where you see only his perceived faults and problems, I see his positives as well. I listed the positive things he did during the game and I'm happy you didn't say anything against that so therefore I'm sure you agree on these points.
Now, you will never hear me say that Kolarov is the best left back defensively I've ever seen. But I believe everybody agrees that a full back must be able to attack as well, just like Sagna did for Sergio's goal, I'm sure you agree. But whereas Sagna does makes the odd assist, Kolarov is a machine for dangerous balls into the area. Sometimes they turn into assists, where we all cheer and rejoyce. But oh, how many times have we all despaired to see a fantastic, hard-drilled "Kola-ball-between-back-line-and-goalie" go wasted, and here the woe should be shouldered by our other offensive players who didn't get on the end of them.
So, when you have a player who is so lethal forward and not great defensively, you have to see what the total outcome and contributions in play of Kolarov's negatives vs positives.
Obviously both Mancini and Pellegrini think the total outcome of his contribution in a game is positive, and I for one agree with them.