Even if someone is being attacked from behind this magic wouldn't really help them in that situation, the attack would probably hit. The disregard for sight is mostly due to the spell reaching out in a ten block radius, this prevents signaling out others and also provides a major risk for the caster. For example everyone around them will have to be transported into the Mindplane and if too many are brought in that'll effectively leave the caster worthless. The mindplane itself also has pre-built structures upon creation, they aren't being transported in.
All the spell really provides is a change of scenery, making a group who were fighting in a city now fight in a dessert and so on. Also using Mental magic on it's own provides a mental link regardless of IC or OOC consent.