Those saying "if you can beige while fortify nobody will ever use it" are incorrect. There are two scenarios I'd use it in:
1. To get the buff on defense to maximize casualties overnight.
2. To briefly bump up my resistance to not get beiged -- how does this work if I can't attack? If somebody's attacking me on the sea (or air), let's say I have no ships, but I have allies who can bomb his ships. I can fortify to bring myself into safety temporarily, then rejoin the battle when the enemy's units have been reduced.
Fortify should be a temporary response rather than a permanent choice, imo.