I'll be frustrated if Chapman isn't a starter this year, however, I think using him one more year as a closer has it's benefits.
1) he's a good closer
2) Brox is still paid like a setup guy
3) Leake is a decent 5th starter anyway and the overall team talent is stronger
4) Arroyo is a FA and Bailey's future is uncertain, so 2014 may be a better time for Chapman to the rotation.
5) Arb-wise, if Chapman can or cannot start, perhaps it's cheaper to find out a year from now when the chips are on the table.