The problem with IG is that it has a very low uptime (15 second on use effect with a 1.5 cd making it's uptime 17% if used on CD), so you need really short fights to have a decent uptime (less than 1 minute). It's not useless of course and it'll do a decent job regardless of kill-time, but it's significantly worse than DSI for example and you're better off using OoV over it for normal length fights (such as the ones you mentioned).