Furball
Evil Scotsman
The best example I can think of is imagine you're playing cricket with your mates, you're playing rubbish and getting heavily beaten, there's 10 overs to go and all of a sudden Kevin Pietersen wanders over and asks if he can field for the last 10 overs. Your team's performance would instantly be much better for KP being on the field with you, even if he wasn't actually contributing anything himself.