IS is useful on a 100mm macro, but its value depends strongly on magnification and how you shoot.

For handheld close-ups that are not quite 1:1, IS can be very helpful, often giving a clear advantage for flowers, general close subjects, and other static subjects indoors or outdoors. At normal subject distances, users report much larger gains.

At true macro distances (around 1:1), stabilization helps much less. Traditional IS becomes less effective as subject distance decreases, and even hybrid systems designed for macro mainly offer a modest benefit at maximum magnification. A practical expectation is that it may still help a little, but not dramatically.

IS only compensates for camera movement, not subject movement. So for insects or anything moving, it won’t freeze the subject; you may still need a fast shutter or flash.

If you use a tripod, IS usually isn’t needed and can even reduce sharpness on some setups, so it’s commonly turned off.

The close-focus light loss you mention can force slower shutter speeds, which does make IS more valuable for handheld work. But at high magnification, front-to-back movement and subject motion often become bigger problems than shake alone.

So: very worthwhile for handheld close-up shooting, somewhat helpful at true macro, and not important on a tripod.