It seems that butter is not good for frying things over high heat
because it can burn
However it IS widely used for sauteeing in many cuisines, and works well.
I looked at the cost of butter, which e.g. in the UK is around £6/kg, and ghee is £8/kg.
Although butter is around 18% water, this still makes butter cheaper, and life is easier if you buy one thing rather than two.
The question then is whereas both ghee and butter will impart flavours to a curry, is there a good reason not to use butter? I think ghee is misssing around the 1-2% of milk solids, as well as the water, and obviously if you wanted to deep fry then butter would be unsuitable (you can, I think, deep fry in lard, at around 160C, but butter won't get hot enough without burning). Or is this possibly a cultural issue, in that butter doesn't keep well in hot countries such as India without refrigeration, whereas I guess ghee is probably shelf stable?