Actually, if you remember from Clone Wars, Obi-Wan was the Republic’s best military strategist who’s experience and expertise they would desperately need. After all, Mon Mothma only asked for Obi-Wan after she and Organa agreed that war is coming. This has nothing to do with needing Jedi as such, and so Kanan and Ezra…

Actually, there’s another Ghost siting that I saw that no-one seems to be mentioning: