2 things. First, he should be a starter, period. ~220 innings vs. ~75...
Second. Since he's been stupidly forced in the bullpen, he's actually been used really well in that role. You want your best reliever to pitch in the most high-leverage situations, and Dusty has surprisingly put Chapman in those spots.
Example. A reliever coming in the 8th with 2 on and no out vs your closer who comes on in the 9th with no baserunners inherited. This referred to as the fireman role, and no is better than Chapman at it this year. He would be less valuable as the closer.
Here's an article from Fangraphs that explains it much better:
http://www.fangraphs.com/blogs/index.php/reds-find-useful-role-for-chapman