Actually I've always thought this....SPOILER..
(SPOILER)Firstly, when sin puts up the barrier it doesn't look like anything that we ever see in the game. Terra Graviton etc involves sin making the attack from his mouth.
Therefore this must a barrier which protects him from heavy fire.
The reason for not using this barrier at any other point in the game is due to the fact that the ancient machina machine actually destroyed the shield at the battle scene - YES!!! the machina does get destroyed - but if you look closely at the FMV you see the machina winning whilst sin's barrier distorts and manages to just gut out and touch the machine.
At the exact point of the explosion we see the barrier dissaperate and then sin tries to make a quick escape back into the sea. I think that sin just managed to beat the machina before it's shield was, in turn, destroyed, this is why he left straight after this point without tearing up some warriers or landscape even!.
Sorry about the length - got a bit carried away!!