I look at it more through the lens of roster construction.
The team, to be good, needs a certain amount of power, HR and doubles. History shows that the vast majority of playoff teams finish at league average or better in HR hit.
To a certain extent, and like it or not, the lack of power is why Kemp was acquired (I certainly understand the need for power but question why it had to be Kemp whose other flaws now have to be accounted for).
Given that Markakis is way below average for his position in the power department, as is 3B, C, SS and 2B and the fact that 1B is now average or maybe a little better at this point (hopefully the hot streak is the new normal), then I can't see any way you can play BOTH Mallex and Inciarte in the same outfield on the same day on a consistent basis and expect to generate enough power to be able to win consistently no matter how good their defense might be.
Once you arrive at that conclusion, then you have to accept that either Inciarte OR Mallex is a 4th outfielder on a good team if they are playing on the same team. If you judge that the talent level of one (let's say Mallex as an example) is no better than being a 4th OF, then no problem, keep them both, start Inciarte and use Mallex as a GP 4th outfielder. BUT, if you consider both potential starters at CF on a good team, and other teams view them that way as well and are willing to give up significant talent in return in trade, then you HAVE to trade one or the other or maybe even both given the right return.
If you are a rebuilding team you don't have the luxury of sitting valuable players on the bench for the majority of a MLB season when they could bring value in return. Teams like the Cubs can afford to do that because they don't have a ton of holes to fill and they can justify keeping a lot of young bench talent (and keep them happy) by pointing to the fact that they are contending.
Now, everyone should note that I don't say get rid of him (either) for the sake of getting rid of him. What I say is if you have two really nice right foot shoes but no left foot shoes it makes sense to try to find someone with a matching left foot shoe that fits your needs and give up the un-needed RF shoe.