It sounds like he does not want to take a definitive stance on the issue or he could actually feel this ambivalent towards the situation. He understands both sides of the issue and does not want to place the burden on any one particular thing when there are so many factors to consider.
Pope Francis has said that people should be welcoming to refugees and migrants while considering the maximum number that they can feasibly accept. While the Pope may appear to be "straddling the fence" on this issue, he's doing so smartly. It isn't the Pope's job to get involved in politics, so he must be as diplomatic as possible when confronted with these issues.
No, by saying "While countries should be open to taking in refugees and migrants in need they also must be prudent about only accepting as many as they can integrate into their societies", I do not believe Pope Francis is straddling the fence. The point he is making is correct by stating that everyone should try and help these refugees but you can't hurt your own country by doing it.
Pope Francis is not trying to straddle the fence. It is not in his character. Francis has been very honest and open about how the church should be more forgiving and open that it has been in the past. He speaks his mind. For this reason, the Pope's position on the migrant issue is his honest opinion for leading the church; not just trying to be politically correct.