I have a couple of 100Ws strobes, and find them very useful as hair lights, or background, or fill, or as finely directed point lights (a snoot is handy on one of these). One reason is because they are adjustable in fine amounts, and they go down to a very low level (6.25Ws). More powerful lights don't go as low (the 300Ws goes to 9Ws, but the 600Ws only goes down to 18Ws). Having fine control is useful. They also weigh quite a bit less (1.5kg instead of 2.3kg) - useful when putting a hair light on a boom over the model (oh, just like you mentioned
I do use 600Ws and 300Ws strobes, though - they are my main and key lights - one needs a bit more brute strength on occasion
So yes, I do see plenty of use for 100Ws strobes.
Bear in mind what coach says, though - my 100Ws strobes have very reliable slave sensors, and I'm glad of that.