Of course this is just my opinion, but I think I know why Blatt gives Harris so many minutes.
Right now the team isn't yet gelling and trusting the system or each other. And the players, especially the vets, and I can say Waiters as well, because of the way he plays, at the minute there is an issue, they go on "auto pilot" mode to their comfort way of playing, and they just "quit" Blatt's system.
Harris is a coach's player, it's in his blood, and I think Blatt trusts that he will continue to work in his system and listen to him, even in dire moments. Which I can't say it's true for everyone else, at this moment…
I think the main remedy for this situation is time - more playing together, and more trusting each other. And this is a lot better that all those issues come right from the beginning and not in the middle of the road, because now the Cavs still have time to resolve the issues and to gel.