The problem I have with it is there's not much room (in my mind) for looking for, wanting, going for the other, more permanent relationships. One is already emotionally bonded/friendly so being a good "friend" and then you add the sex; why would either person think they wanted/needed more? It's a relationship any way you slice it. You can't be a friend to someone and not care about their "feelings". Sex doesn't cause the feelings and what feelings come of it have to be suspected or the sex isn't any good.
I think it is an opportunity to fool one's self and/or potentially get hurt or hurt a "friend". Either that or my husband is a friend with benefits (beneficial friend?)