I guess I would say two things.
First, when you look at Heisey starts, they have been occuring on days when most of the Reds are not hitting, so it could be argued that he is playing on days when the other pitcher seems to have their best stuff. I recall Heisey being only the second baserunner (in the 7th) against the Padres a couple of weeks ago before scooring the tying run in the 3-2 comeback. Last year I remember he broke up a no-hitter late in a game.
The second thing I would say is that, it obviously takes a different approach to pinch hit than start. Juggling back and forth would be difficult. I would like to see him get a real prolonged chance to start and see what he can do. I don't think anyone would argue that he has been given a real chance to succeed as a starter. We know what Gomes can do. We know what Lewis can do. Heisey has not done worse, let him have a chance and if he fails then you can look at other options. He has shown he can do it in the minors and in the spring. Let's see if he can do it in the regular season. There seems to be no downside to giving hiim a shot.
If the Reds don't give him a chance, some other team will and they might regret it.