Any fan that cannot see both sides is going to wind up being wrong way to often. IMO, there are way too many considerations for a straight-out yes or no.
The age of your trade target
Recency factor for your target
Trade value v projected value for your prospect
The need & fit for your prospect
The recency factor for your prospect
Monetary considerations
The guys I have the most interest in trading away:
Duran-Great value but very poor fit
Any of our #8-infinity SPs. No matter what the talent, they are unlikely to ever get a chance. Someone like Harrison is going to get de-valued quickly if he is sitting in the BP, but has starting value to someone else.
Campbell-The recency factor hurts him, since he struggled. But even if he succeeds, we still need an opening, TBD.
Casas-Bad recency factor, but if we are going to sign an Alonso-type, we need to establish a trade.
And since this is about Mayer/Marte, I'd make Mayer my opening day 3B and never look back. And I would factor in that Marte likely only has 1-2 very good years left. Which is fine, since he is cheap. But my guess is that his contract still only has a surplus value of $36M. And of course, the second rule of holes is that you don't dig a second hole to fill the first hole.