I completely agree that if DC isn't doing it in practice, he shouldn't be playing. No one deserves a playing spot by default because they're 7-foot.
It's not the best for DC or for the rest of the players. If he's not working hard or executing well enough in a gym with the team alone, how can you expect him to execute in front of a huge crowd or in a hostile environment?
I bet the coaches are frustrated with him because he's got potential to be a lot better, but he's just not doing it. Think of how much better he would be if he did start doing the right things in practice.
Rashad Austin seems to be taking more of the available PT. Good for him. He's working to earn it.