I don't think we understand each other tbh.. when you talk about weakest link I read getting rid of him is the priority of the Spurs organization but here is the thing it is not. Spurs priority is to add shooting and to deal with Sochan headache for a start. Barnes is a serviceable role player never injured with a 3pts shooting that opponent needs to respect, he brings enough spacing for our guards and Victor to operate inside. We have 3 guys who can do that: Champ, Vassell and Barnes and we have seen opponents not respecting a lot Vassell 3 pts menace eventhough this year one cannot complain, as for Champ opponents are paying attention but it is quite recent. I don't recall a team just daring Barnes to get hot from 3s.
I have nothing against putting Barnes on the bench but Champ is already starting, Vassell is injured who are you going to use ? Harper ? Ingram ? DJG spacing that is already not good will get worst. The only option would be KJ who is shooting 41% from 3s this season but if you do that you penalize deeply the bench minutes production. I like KJ as a 6th man role.
Also you can laugh about + and - on a particular game... fine but Barnes on/off are consistently pretty good and it is not by chance looking at sample size.
Finally I don't get your "name one player..." you are comparing bananas with orange, cannot compare roles in a vaccuum. We have 3 guys capable of providing spacing Champ, Vassell and Barnes. This year Champ and Vassell are clearly better but was not the case last year besides we don't have enough of those guys, let the Spurs bring more then yes you may say he is the weakest link.. not the case this year even it is much better than last year.