How is it going to work, if a Manager wants to bring on a Sub who scores Penalties for fun, or a Goalkeeper who saves them for fun, during the 90 minutes? They might not be the best Player or Goalkeeper, but the Manager can't sub them with the original Player. THEN they're going to play an extra 30 minutes?
Look at the Liverpool v Middlesbrough League Cup game, from earlier this season. 30 - 29 to Liverpool in Penalties, and then play another 30 minutes. If I'd been one of the players on either side, I'd have been sent off for dissent, because I'd have told the Ref to "Fudge off!"
Perhaps change the rules and have away goals count double, at the end of extra time. That gives both sides something to aim for. 1 - 1 at the end of 90 minutes - 30 minutes of extra time. The home team scores and win 2 - 1. Or, if no-one else scores, the away team are awarded the game 2 - 1.