In basketball, replay is mostly used for the clock, but also to determine whether a player's foot was on the line during a three-point shot (only used on close calls obviously). In hockey, they'll use it to see if the puck crossed the goal line, or if a goal is scored right as time is running out in a period. Hockey can use to make sure a goal is good for any number of reasons. Like touched with a high stick. Intentionally kicked or punched in. If baseball had instant replay I could see about 25% of all double plays being reviewed. In that case I say that there are times when instant replay would be helpful, but because of the number of unnecessary reviews I say it's a bad idea.