A curry that is not almost covered in oil when cooking is over is not, to my mind, likely to turn out well. "Cook until the oil comes out" is a very old curry apophthegm, and if not sufficient comes out, I add more.
Honestly Phil you've already stated the best method of obtaining spiced oil which is to liberally douse the curry you're currently making in oil and spoon off the excess at the end. Instant spiced oil, absolutely no faffing required. And as you rightly say, a curry cooked with niggardly quantities of oil is demonstrably lacking in the taste department. So you're killing two birds with one method.
I've been doing it this way for years and I'm pretty sure we'd find some reference to its importance in the early days of the forum. Only works when cooking savoury curries though. Trying it with a korma would be a very calorific disappointment as the oil would never separate enough.
The depth that this oil adds to a curry is unmistakable so I'm at a loss when people say it adds nothing.
By the way, apopthegm? That's a new one on me; I'll stick to aphorism, it's easier to spell!