I disagree to a point. A trade can be defended because:
1: The idea was fundamentally sound, and the player faltered for reasons unknown at the time of the trade. An example here might be a player who is traded for, and then becomes injured and ineffective. An obvious exception to that is if the injury was predictable, or one that was a known risk at the time of the deal, such as the Bailey or Gagne trades.
2: the trade was an attempt to seize a championship and, whether the championship was or was not itself seized, the player lived up to his end of the bargain. I didn't like the trade for Victor Martinez, but it can be defended based on the fact that he played up to his ability in Boston.
3: The reward turned out to be low, but little risk was taken in the transaction. This could mean that the trade return wasn't effective but also wasn't necessary or expensive, such as in the Gagne trade, or that the outgoing pieces had no place on the team, such as in the Martinez trade.
The reason I'm down on the Reddick for Bailey trade far more so than Lowrie for Melancon, is that the latter trade was ultimately in category 3. We really didn't give up all that much. As much as I liked what Lowrie could have done if healthy, he wasn't. And really, while Melancon struggling a bit more in the AL East than he did in Houston was predictable, the order of magnitude of the disaster really wasn't.
Bailey on the other hand, we gave up a player who had already played at a high level for us, at a position that wasn't securely held by anyone else, to get back a player who was under control for less years and had well documented injury concerns going in, you don't get any pity points when the coup de fail is complete and the guy actually gets hurt.