People cannot have it both ways... whether he is a system player and you have to measure his value within a system, in that case most advanced stats are useless or he is not.
Advanced stat is usually a measurement and ponderation of all aspects of the game, Parker plays in a system where Pop asks him to focus on his strengths: setting offense, penetrating to open shots for 3 pointers specialist, scores in the paint etc... his TS% is affected by the fact he doesn't draw a lot of FTs because he cannot outpower bigs when he goes for layups, he needs creating space and avoid contacts, his assits pg suffer too on the way Spurs play their offense. Spurs don't ask him too to be a 3 pts scorer neither ask him to rebound the ball, all these elements give poor advanced stats compared to other players.
Only an idiot would consider Parker a cancer or a dude comparable to a bunch of scrubs sorting and parametrizing advanced stats.
Fact is Spurs won 4 rings with him having one of the biggest usage rate of the team, he adapted his game from 4down to ball movement, he is a key piece of Spurs historical run last 14 years and had been extremely valuable for Pop's system and Spurs success, like it or not.