I'm guessing he meant jacket-style (traditional) BCD, which would act like a life jacket. As stated by
@jagfish that the back inflate would tend to push yourself into horizontal position, which is good if you want that way all the time underwater. For me if I'm in trim position all the time, after 45-min in that position, my neck hurts. It's like walking on a road and looking up to the sky for 45 minutes. I like to go vertical once in a while.
On the surface, with big waves, and waiting for a boat to come to pick you up, I like to just float vertically, motionless, which I could do with the jacket-style BCD. With back inflate, you have to lean back or finning backward all the time to keep yourself vertical. That could be tiring after a few minutes waiting for the boat to pick you up in a big wave situation.