[WarBringer]

In my opinion, Gandalf bring Pippin with him is just another part of his plan to defeat Sauron. It was said (either in the books or the movies, I can't remember) that even the wisest cannot see to all ends, so perhaps Gandalf believed Pippin had a part to play at some point in the battle, even if it was hidden from him at the time. In another words, Gandalf brought Pippin with him believing that he may have something to day in the battle to help the cause of men.