When ever I have spooned off excess oil from a curry, and used it on the next one, I believe that it most definitely adds more depth of flavour aswell as filling the room with that restaurant aroma!
Ray, me old fruit, where have you been hiding? I've missed that bit of banter we used to have! Hope all's well with you and family etc..
And yes I agree with you here.
Whenever I've used spooned off oil in another dish, a Saag bhaji in my example, which is a relatively simple dish and easy to compare with one without reused oil, I've noticed additional depth of flavour added, but it was subtle and certainly didn't transform the dish into something else.
So my conclusion is much the same as yours, it definitely adds an additional layer of flavour but, and it's a big but, it didn't do enough for me to warrant spending lots of time making up large batches of it.
I suspect that BIR's do this out of sheer expediency, practicality and without wishing to waste all that flavoursome oil. They scoop excess oil off a dish and sling it into the base sauce. Rinse and repeat that a few times and you have lots of additional flavour, they probably also add a new pot of base to that same pot.
It would be interesting to try this ourselves just to see the taste results.
Make up a batch of base, make a dish, skim off the oil, sling it in the pot of base, freeze the dish and number it No.1. Make another dish from the base, skim off the oil, sling it in the same pot of base and freeze the dish and number it No.2 and so on till you've used all the base up.
Then taste the results in time and see if there really is a difference between No.1 dish and No. 4 or 5 or whatever.