ims is easily cancelable, especially while comboing in stage with lots of other mobs shooting and running at you. nothing really survives basic shift combos at that level anyway. u wont even get to the end of ims at the end of most combos if you know how to combo. plus no ims combos allow for easy(er) dodging.
on fast attack bosses its fairly useless. to get its max potential you really have to get all the hits in and thats never a guarantee. its animation is relatively long and its not cancelable. so lets say your at rigel and you ims and he does the dragon then ur dead (at least you looked cool).
its only real advantage in stage is as you mentioned: The hax dmg multiplier, but thats moot since most stuff dies quite easily anyways.
not to say it's bad. i just think u give it to much credit at 9/10.
PS: i have lvl4 ims and liek it cuz it's liek OMNISLAAAAAASHHHHHH