It is pretty really damn useful, really. Not so much in the difficult boss fights, but I used it on the ultra-final boss a lot before I figured out his pattern.
Like, you can't really use it on the 2nd form of the final boss because you NEED to be attacking him all the time or you'll just die too fast, but for the 3rd, 4th and 5th forms it is pretty useful.
Same for just like SWIMMING AROUND finding treasures, it's nice not to have to worry about getting hit at all.