You can get a SRO much lower when it's milled into a slide, which mine is not. The very bottom of my optic is level with the top of the slide, about 0.150" taller than most mill jobs. My current setup is insanely tall relative to what most people have with their SRO or DPP equipped carry guns. It really isn't very practical for me to put BUIS on the CO gun. I don't see this as a binary "must have BUIS always" or "BUIS don't matter" thing. Just that after looking at how much of a pain in the ass it would be to put BUIS on my CO gun, I decided with my specific setup that it wasn't worth the effort. If I shot a different setup the cost benefit analysis could be different.
In USPSA I don't think it significantly matters which way you go with or without backup sights. I realize 99% of people don't use them but I think that is also somewhat of a cultural norm. How many CO or Open shooters have actually tried using BUIS on their guns to see if it even changes their performance to have something at the bottom of the window?
I don't care if anyone else uses BUIS on their carry gun or not but personally there are shots I don't feel comfortable taking without any sights, and therefore I want sights when I can get them. Didn't mean to imply someone is wrong for not having them BUT I hope that if someone is not using BUIS then they understand what shots they can and can't get away with.