Think of the TCR series option as a 2nd chance for local runners to get into the race. It was put in place many years ago when the race suddenly filled very quickly, taking many long time local runners by surprise.
Also, due to the requirement that you can't miss one of the races, the TCR w/ PPA option fills very slowly. Personally, I'd only register for the TCR series if the PPA was already full. In John Vomastic's case, if he was trying to get into the PPM and knew that he would not be at a computer until later in the afternoon then I'd use the TCR series option. For the PPA I'd roll the dice since that usually will not fill up for a week or two, and even if it does then you can fall back to the TCR option.