It's a good question, and one you can really only post outside of the DIR forum, where the desire to be the same as your teammate is not only assumed, it is the only acceptable answer.
When my husband and I got certified, we picked out our equipment to match, so we would each understand the other's setup, and parts would be pretty much interchangeable. It made sense to both of us at the time, although NOTHING we bought would be remotely DIR. We had back-inflate BCs, integrated weights, Aqualung Titan regulators, Air2s . . . and the thinking that caused us to buy everything pretty much the same still seems valid to me, ten years later.
The difference now is that I don't just want to be like my husband -- I want to be like a whole POOL of dive buddies, so that we can grab a buddy at random and the same ideas still work.
I have always wondered why there is so much resistance to the idea of standardizing dive gear configuration, especially at the recreational level.