Then why would you want Dunn on the team?
Hamilton is your starter in Center the next 2 years if Dunn is on the team.
Griffey is your starter in Right and he's not going anywhere the next two years.
Why would you want to pay $13M more next year for Dunn when you can get almost equal play from Bruce.
$13M is much more wisely spent.
....and to suggest that Bruce would be the 4th outfielder is a good way to ruin a Bruce's career. The Mariner's did it last year with Adam Jones, and they're doing it again this year with him.
Now, I could see Dunn playing the first half of the year with the REDS, so they could take the time to trade him, and Bruce could stay in AAA for the first half of the year playing every day, but he's going to have to start for the REDS eventually, and that means taking Dunn's spot, not Hamilton's or Griffey's.
Griffey puts people in the seats. Dunn doesn't.
$13M could be much better spent, and I doubt if John Allen will allow it to be wasted on the difference between Bruce and Dunn.