Annie Modesitt is a combo knitter and says that your ribbing is sometimes faster because of the way that you purl. and perhaps tighter because the purling makes the purl st the same size as the knit st.
Combo knitting is done flat because when we knit Stockinette flat we knit into the previous purl sts and whatnot.
I tried it, too, and found that I was slower as I knitted, I guess because I was getting used to a new way of knitting. My ribbing looked about the same, too. I'm sticking with my tried and true conti knitting