As 'interesting' as this thread is, it may be irrelevant as, as we all know, taste is subjective. One person's perfect blend of spices is another person's Trump.
When I used to make more traditional curries, there was nothing better than dry-roasting spices and grinding them. The aroma and flavour was, and is, so fresh and rewarding. Having reached a stage in my BIR journey where I am happy with my (subjective) results, I am tempted back to more, dare I say, creative methods of producing dishes of Indian/Pakistani/Bangladeshi origin.
Going back to the use of commercial curry powders in BIR spice mixes, I have always avoided 'curry powders' as I have found them to be bland and 'industrial-tasting'. Why would you include, in your spice mix, another mix of spices which you could include yourself? Having said that, I contradict my own argument by using garam masala in both my spice mix and in my base. I fear self-flagellation is the only answer. I think, subjectively, that my own taste prefers the more fragrant flavours of garam masala to the, in my opinion, more bland flavour of commercial curry powders.
A spice mix is a convenient way of making a quantity of ingredients which can be used to make consistently spiced dishes in haste; which is one of the hallmarks of BIR.