If there's a mando, then you need to watch the disc (that falls under watching it for compliance with the rules)....as I said in a previous post, the PDGA really can't make a rule that you have to watch the disc to see if it goes OB. Lots of holes have OB out of the line of sight from the players. There are dogleg holes with OB, there are holes that have elevation changes with OB; and in both of those cases you may not be able to see the OB from the player's lie. So are you going to make a rule that all the players have to run up the fairway after each throw to see the entire flight in case the disc goes OB? That sure would be fun to see.
Seriously, a mando is always within sight of the tee pad, but OB isn't. I've seen tournaments where a disc was thrown, went OB, but no one saw WHERE it went OB, so the card decided the most likely spot where the OB happened. It's a fact of life that not all OB is observable from the spot of the throw, so mandating that you have to watch the disc to see if it goes OB isn't going to happen.