Baron throws more games than it wins.
When you are ahead, baron is not needed. It is a false objective where the only team that needs it is the team that is losing. If you are ahead only greed will make you go for baron since you have enough power as is to be ahead of your opponents. Sure the gold and buff help keep or push that lead, but if it gets stolen, you could end up losing the game as a direct result of it.
So make sure you are 100% positive that, when you are ahead, it is a free baron or you can survive it getting stolen away.