Greets, Iain. Well; there's always a tension between the two schools of thought-'what tastes best', and 'what stays closer to trad. DIY BIR', as Phil puts it. Ghee is an excellent example. It'll almost certainly improve the flavour of any curry, but, conversely, is more expensive than veg oil and so less likely to be used in bulk by all particularly except high-end establishments.
^^^as is my understanding of the UK situation anyway.
^^^as is my understanding of the UK situation anyway.