Tenn raises a good point in that it's hard for us, as fans, to to judge him because we see him play so rarely, and sporadically at that. That said, I'm guessing the coaching staff have more data points in which to judge him, both in games and scrimmages, and they've probably been judging him all season, and this is what they've deemed him to be. What we might be seeing is the result of their judgment, not necessarily an audition i.e. "prove to us you deserve more PT". I think they've long ago decided he's not got what it takes to be a regular rotation piece like a Hauser or a Payton who can shoot the ball, but for $2m they're willing to ride with him because he might be a plug in piece when others are injured or if they need some garbage time minutes to be taken up.
Also, with respect to him starting, on the body of evidence over the past three years, Joe strikes me as the kind of coach who doesn't like "cascading changes" - where if someone on his starting lineup is out, he would rather plug a new person outside the regular rotation in there, rather than promote someone from his 2nd lineup, which would then disrupt his second lineup. That's probably why he put X in to start, along with the fact that it was probably a nod to him playing for Memphis and a chance for him to play in front of family and friends in a game that was the end of a long road trip. I've noticed he does this, putting Torrey Craig to start, or Jordan Walsh, rather than promote someone from his second lineup and disrupt the combinations that those guys have when they come on.