Solus,
For me it is a comfort thing. I carry IWB 99% of the time. However, when I'm not wearing a T-shirt the leather against my bare skin, and there is a lot of it, can be uncomfortable. If I don't like the feel I just move the gun outside the pants but still under the belt.
BBWB negates the benefit of IWB, and with the holsters I have it adds no security of the gun over OWB.
Personal preference is what it comes down to, but that is 110% of all carry choices isn't it?