Moon Barrier can be very useful if applied correctly (although I'm sure it'd be very frowned upon by people and deemed cowardly). You could set up a barrier in order to gain a major environmental advantage considering it'd eliminate any straight line of attack against you. Not only does it cause obstruction, but it also negates a lot of attacks, such as Fire Bomb, Fire Orb, etc.
Moon Barrier takes a little practice and adjusting to use because it's more of a way to make someone go where you want in a defensive sense. You can set up a Moon Barrier so your opponent can approach you from only one of two sides, which can allow you to easily move away, or catch them while they're moving there. Since you can also go through Moon Barrier, and your opponent can't, if you position it so your Barrier is behind you and your opponent is in front of you, they cannot pull a full straight combo since they will be obstructed by the barrier and you'd travel through it.
These are just a few uses, and I'm sure there's more. Primarily a defensive spell in PvP but it can be very useful when used correctly. Just...get ready for some whining.