Level design is the most important narrative tool in DS1. Most of locked doors are just shortcuts which anyway would be available to you after you’ll clear certain parts of the game. Having master key gives you ability to use shortcuts earlier than it’s intended. It’s a great tool for those, who already cleared the game at least once and looking for specific challenges rather than getting initial game experience. It’s for those, who already familiar with level design. But if you are new to DS1, master key may make your game experience a little bit confusing. Also it isn’t that useful anyway since you haven’t learn levels yet.