Only 45.45% of the time are they boosting you, because there are 6 slots on the enemy team and only 5 on your team. That means they are more likely to appear on the enemy team.
The same things, however, is true of throwers. And smurfs must throw in order to maintain their low rank… so you could say it is a net 50% that they cause you a win or a loss.
The thing is, if you would otherwise be climbing with a 55% win rate, and half your games have a smurf (carrying or throwing) in them, you now have a 52.5% win rate, and half of your games are unpleasant stomps that you have no control over.
You can argue all day about what percent of games are actually smurf/thrower games, but regardless of what that number is, nobody wants their climb slowed down by any amount of unfair, unfun stomp games.
If you disagree then go ahead and tell me about how very happy you are to get a thrower on your team because you know that statistically throwers are more likely to be on the enemy team.