The problem with Turner is twofold. One, he is bad defensively, so he needs to be a major plus on offense to justify PT. Second, on offense he needs to have the ball to be effective, but he isn't super effective with it, either. That's okay in our current lineup, as we don't really have superior ball-handler options (except now for IT), and it's okay in a bench role - but it's not like he'll ever be a 6th-man-of-the-year candidate, either.
In contrast, Pierce was good defensively, he was good off the ball due to his shooting prowess, and with the ball in his hands he was outstanding due to his awesome ability to get his shot off or to draw fouls. I see the analogy, as they're both wings with PG skills who don't thrive on athleticism, but PP is just SO much better that it's hard to compare them.