As a game developer, this is a little scary if you are developing any custom content for the optional areas of the game. Why would you spend all that time and effort making something that maybe only 10% of the players will ever see? If game-players were living in a vacuum, that would be a much bigger deal, but we gamers – contrary to how the media likes to portray us – tend to be pretty social. We talk. And we love games that allow us to compare different experiences.
But how much of that experience should be “optional?” The approach even among the classics are pretty widely varied. Even the games we celebrate as being so free-form typically had very little “interesting” content that was completely optional. If you found a dungeon in the game, there was usually some reason you’d have to visit it at some point in the game in your chain of tasks leading to the conclusion. That is, unless you have spoilers handy and are able to bypass certain vital clues / passwords.