There is no reason that why he should not pose for selfies. Its a free country, if every ordinary person has the right to pose for a selfie then the same applies for the President as well. Although any sort of abuse should not be tolerated but a sweet looking selfie would not do any harm to anyone.
One of the things that I like the most about our president, is his ability to interact spontaneously with everybody, especially with ordinary people. I think that being the most important world leader has not affected his humbleness and he is still the same funny kid that came from Hawaii to Illinois.
In an ideal world, the President would be able to pose for selfies without further repercussions. He's a man with a sense of fun, and it would be great if he could express that and let people know he's human, without it going any further. Unfortunately, it leaves him open to people taking pictures they'll then use against him, so he should be more careful and avoid them.
I don't think the President should pose for selfies. His position is one of great responsibility and he should always be aware of his behavior and how the public perceives everything he does. I'm sure he enjoys a chance to let his hair down and just be a regular guy, but he doesn't really have that luxury while in office.