I'm not saying that I wouldn't like to have that amount of value per vacation day, but to the extent that a 7-day vacation would then be worth 1/2 of an average month, how is that different from simply adding a proportional number of vacation days in a year?

Either way, you're getting more total vacation hours, and an equally greater amount of time off vis a vis less flying assigned throughout the year due to vacation.

The downside of more vacation value per rather than more vacation days is that reserve pilots' on call days are prorated by the number of vacation days they hold, and not by the value of those vacation days. IOW, while a regular line holder would get more days off in a vacation month either way, a reserve pilot would only get more if he had more vacation days.