Pro #2: He fills a need
Outside of Mike Yastrzemski, there are no players guaranteed a spot in the outfield to begin the season.
Therefore, it would make some sense to bring in another offensive-minded outfielder to give Dickerson, Duggar and the rest of the outfielders in camp some additional competition.
The question remains whether Pence could still reliably play in the outfield, but if he hits like he did last season, then it may well be worth adding him to the roster.
To account for his defense and to keep him healthy, Pence would fit best in a platoon role, so he would have to be willing to accept a lesser role as an outfielder over a potential everyday spot as a designated hitter.