Martin looked pretty lost on one of the balls off the wall against Boston, which was for extra bases. Minnesota's other regular outfielders are not exactly gold glove material either.
The fact that Outman (it is a deliciously ironic name for him) is still here tells me the Twins have answered your question. The other issue that wasn't raised is a fourth outfielder is going to play how often? Sure, if Buxton is going to become a semi-permanent DH, it provides more regular playing opportunity, but it is worth recalling that Mendez started the year in AA ball. He's 22 and can use regular playing time.
It will be interesting to see how the Twins deal with their glut of OFers IF Jenkins and Rodriguez can stay healthy, not to mention Roden - and you also have Gonzalez and now Mendez. At least one of these guys has got to be in the thought process for 1B, right?