There are many many many CONs, but the one that comes to my mind is that it is NEVER possible for 2 humanoids to have EXACTLY the same feelings (or lack of feelings) on the subject of having sex with each other. NEVER.
One quite obvious con would be if someone started to develop feelings for the other person and those feelings weren't reciprocated. It would mean devastation and a permanently broken friendship.
Another con would be the loss of self-respect and respect for the other person as a by-product of that 'relationship.' Not only that but the feelings of not being worthy enough for real love when it comes by.
Those are some pretty serious and intense cons there.
Now shush. It's sleeping time.
The cons of having a friend with benefits is falling for the friend and not having them fall for you. You want to continue the friendship and the "benefits" because of the way you start to feel, but at the same time you want to end at least the "benefits" part so that you don't continuosly keep falling deeper and deeper.
There is no such a thing "FRIENDS" WB- if sex is in question or money; one part is always emotionally involved - otherwise they would not even start the "relationship"; its common fact that man start relationship couse of phisical atraction to woman and in time if sex is satisfying they "fall" for a woman; with woman usually is the opposite - she fall for a man in a first place (either she is aware or not); we tend to clame that we are free and open and want more open relationships but in fact we all look for one thing and one thing only - SOUL MATE, just it seems that nowadays term soul mate implies more then just love as it was in old times; Thus - con is - WHO PLAYS WITH FIRE, GET BURNED! my 2 cents!:)