The problem comes up when there isn't a face in the group, or when you're split up and both groups still have to be able to do things like lie convincingly, or other such things. If you don't have a face in the group, but instead someone who is acting as a face as a secondary or tertiary role because they have the highest CHA, you're uber-boned. And that happens more often than you might think, especially when people drop out unexpectedly like they do in online games. Things tend to get very pink mohawk at that point. Which tends to piss off DMs.