More about chemistry and fit rather than points.
St. Louis is a great player don't get me wrong, i love the guy and think he's a prime example of a little guy succeeding in the NHL. But should he really be on the roster playing in every single game? I don't think so. Like i said, he'll most likely be on the team, but he probably won't play the really important games. Maybe against like Latvia or some country like that. Games against Russia/USA, i honestly would prefer someone who's more feisty and gritty.
To beat the Russians you need to be in their face and piss them off. If it was purely a game of skill, the Russians would probably win easily. The Americans are the same. Low-scoring, lots of trap play and lots of defense. Can Marty handle it? Some would argue that he can, but i'd be more comfortable with someone like Perry.
To me, the 4 RW spots should go to: Stamkos/Nash/Perry/Carter/Sharp. I don't really know if Carter should be on the list since i don't know if he plays LW or RW.. But nonetheless, I don't really see where St. Louis fits. Unless he switches to LW? I don't know, just don't see where he fits.
One thing is for sure though, if he isn't in the top-6, he is relatively useless compared to other players out there to fill the "grinder/defensive" forward positions.