Price, Moore, and Lynn were developed to be starters for years in the minors. Their bullpen stint was short and they were inserted into the starting rotation the next year. CJ Wilson failed as a starter in the minors and they converted him to reliever where he did better. So he was stuck being a reliever due to that fact. Eckersley spent 11 years as a starter. Jeff Samardzija has 86 minor league starts under his belt, was sent to the bullpen since he wasn't that great, and now forced into the rotation (he is 8 starts in, let's hold off on calling him a success just yet). All of them have one thing in common, they were given time to develop as a starter. Only Wilson and Samardzija stayed in the bullpen past a part of a season, and that is mainly due to the fact they were not very successful as starters in the minors. Plus, they were told they were going to be starters prior to the start of the season. How many suddenly transform into a starter? You can't expect Chapman to suddenly do that this year, and it's pretty clear having him as a 5th starter (assuming he could succeed there) would be way more valuable. Yet the Reds don't know how he will fair, so more than likely he is stuck in the pen for the whole year.
The Reds needed to let Chapman develop as a starter and determine what he is. Instead they rush him into a reliever role and he is getting stuck there. The bigger question isn't how many guys went from bullpen to starter. It's how many guys were given zero chance to develop as a starter, rushed into a bullpen role without knowing if they could start, and then kept there out of need in the bullpen at the start of a season. That list is very, very small. Anyone with talent like Chapman is developed as a starter and only ends up in the bullpen for any extended amount of time due to being poor at starting.
I don't care about the few games Chapman may have saved this year. I care more about the fact the Reds are 2-5 in games Leake starts. If the Reds had continued developing Chapman as a starter they may have been able to turn that around in a second. That is worth way more than a guy that gets in an inning every other game, and clearly has issues with throwing on back to back days. The Reds are now pretty much stuck with Leake as a starter, and unless he turns it around that will hurt a lot more than Chapman helps in the bullpen.
I am not worried about Chapman possibly being a starter in future years. That is not out the window. Yet for this year it certainly looks that way, and the way Leake is going the Reds are not going to win anything throwing him out every 5 days. As mentioned, you can get a 3 ERA bullpen guy for a sack of used jockstraps at some point this season, which will be a difference of a couple of games vs. what Chapman gives you (unless he somehow stays at a zero ERA all year). Yet good luck finding any decent starting pitching that won't cost an arm and leg.