I'm a personal fan of either taking Baron as a means to break through to an inhibitor, if we have stronger teamfighting but can't safely dive, or after the third inhibitor goes down in order to ensure that even if we go 3 for 2 in the fight at their nexus the survivors just need to juke and stay alive long enough for the minions to win.
When using it to end the game, it's more of a security policy than anything else. Like elixirs -- we might be ahead, but the buff from those is still nice to have. Same goes for Baron and 5th Dragon. If you can safely get either of those, you probably didn't need them in the first place, but if you can stack advantages why wouldn't you?