On the whole, I agree with you; it's a mistake to ape pronounciation that doesn't come naturally (and especially to try to sing songs in languages or dialects you don't understand). The "instant gratification" people don't agree, of course: "You have no right to tell me what to do", and other tedious stuff of that sort.

Mind you, I rather think that Martin Carthy's "mispronounciations" of town names, if such they are, probably reflect the way they were pronounced by his sources and may be perfectly legitimate. He's generally quite careful about such things. Specify a few examples and I'll give you a considered opinion.