As you might already know, there have been alternates designated for the World Tour Finals for many years now. It's not really a new idea. The main difference between the World Tour Finals and every other event is the round-robin format it is played with versus the single elimination format that every other tournament on the calendar plays under. With a single elimination format it's simple - you withdraw due to injury or retire mid-match, then you lose, and your scheduled opponent moves on to the "next round". Since you can't do that with the round robin format, you need alternates. Also - alternates serve a purpose for the other players in the draw - since those players advance to the semi-finals based on their combination of matches won, sets won, and games won during group stage play. So they need the opportunity to play all 3 scheduled matches to see how their results can (evenly) compare to the other players in their group.
If you reference page 40 of the ATP Rulebook, it speaks about how alternates are only eligible to receive points and prize money for their play, but they cannot advance out of the group stage play unless they replace someone before that person plays their first match - in which case they are made a direct entry. Ferrer does not 'inherit' any of Raonic's wins or losses - only his own from the matches he plays in.
Also, just try to imagine all that goes into an event like the World Tour Finals - the scheduling, the ticket sales for each session, the television broadcasting schedule, everyone else involved (tournament staff, ball kids, etc..etc. There's a reason the World Tour Finals have alternate players on stand-by so that even if a player (like Raonic in this case) withdraws from injury - someone else can step in and fill his place and everything (I mentioned above) can continue to run smoothly. Even with it being tennis -it's still a business underneath it all. They don't want to just tell all of the ticket holders "sorry but the guy said he can't play" if they can bring someone else in that's (usually) just as good.