As an isolated move, signing Alejandro De Aza was a fine decision. Starting him in the outfield, or at least using him as the larger half of a platoon with Juan Lagares, was more than understandable. And it only cost the Mets $5.75 million. However, all of this was before Yoenis Cespedes miraculously fell back into the Mets' lap. The Mets—a team that already has a capable backup outfielder in Juan Lagares—can choose to trade or keep De Aza, as other teams have interest in him.
While he made his major league debut back in 2007, De Aza only began to get regular playing time in in 2012. He's been remarkably consistent since then, with wRC+ marks of between 96 and 106 in each of those seasons. Defensively, UZR seems to peg him as roughly league-average as well. In other words, De Aza is, more or less, the definition of league average. But the projections seem to deviate from each other pretty significantly on what type of player he'll be going forward.
As long as the Mets don't trade him, De Aza will enter this season as a bench piece. He probably wouldn't be the first off the bench as a late-game defensive replacement, in deference to Lagares, but De Aza can still provide value in other ways. Besides the insurance that he provides, he can also serve as a pinch-hitter or pinch-runner. He makes a great addition to the lineup when the Mets need another hitter in interleague play. He also has significant experience in all three outfield spots, making him a perfect extra outfielder.