Our company is certified for BRC and IFS and we are even waiting for GlobalGAP to be introduced for processed porkmeat. It's purely costumer driven.
For myself I only ask for at least one GFSI recognised scheme from my suppliers. But for example, my supplier of spices and herbs, who is only IFS certified, I had to do a supplier audit for food fraud as this is not so predominant in the current version of IFS and this is my red flag raw material. But as it is my red flag material, I probably had to do the supplier audit anyway. But what we saw during the audit was that this supplier was not very aware about the difference of food defense en food fraud. Without our own audit, we would not have gotten enough information/prove on their food fraud system for our BRC-system. Focus there was mainly food defense as in the current IFS and less to food fraud as in the current BRC.
So I can understand why customers ask for a certain system, depending on what your biggest risk or focus is. But your customers tend to do their own audits anyway as well, being IFS higher level and BRC AA grade certified or not. And I am not sure if that is good or bad, but you could definitely question the need all for that.