well the isu has no rotating turret, and can be very easily taken out with a panther. and once you got your opponent on the defensive with his isu your infantry can move up much more safely. Alternatively you can just have your infantry march to anywhere else on the map as the isu is also slow as balls.
1v1 and 2v2 maps are much smaller than 3v3 and 4v4 maps, so "just ignore it" is not a viable solution.