Earlier quoted context omitted.
The good that could happen: comp increase, or promotion. Or even just making sure their manager is aware that the entire dept productivity is on OP (makes their leverage obvious). Or, if the expectations are low, OP could de-stress and kick back at work, improving their life. Possible bad: gets the other person fired. Puts a target on OPs back. Point being, this will fester if it isn't addressed. Fairness is importan…
The good that could happen: comp increase, or promotion. The odds of that happening are approximately 0. More likely the "slacker" guy is allowed to get away with it because of nepotism or some other factor, and calling him out with result in negative consequences for the OP. Very, very little good usually comes out of speaking up about stuff like this.
Second, for some people, fairness is important. If OP is one of those people, they may prefer knowing if their employer shares those values. If they don’t, then OP may want to know with certainty so they can work elsewhere.