Personally, if I were spending 12 days at WDW, I'd be taking not one, but two (maybe 3 or more) day-trips to Kennedy Space Center, and most likely going to Adventurers or the Warehouse those evenings, so 10 days of park admission would work out just fine for me.
With my current plans for this November, I'd be deliberately shorting myself for the day I intend to spend at KSC, just to cut costs. With the MYW system, it just doesn't make any sense to buy a day's admission for a day spent elsewhere, particularly since I'd have to pay extra for PI, more than I'd pay for just one evening's admission.
Consider the following scenario (based on my 2002 trip): I'm spending 8 nights at WDW, including 2 day-trips to KSC. This adds up to 7 full days and 2 partial days.
The general assumptions are that I'd want the park-hopper option, and want to spend my arrival evening and departure morning in theme parks, and that I'd spend the evenings of my KSC days in the comedy clubs of PI, but that I have no interest in the water parks or DQ.
With the MYW system, if I wanted to do everything on my MYW, I'd have to pay for 9 days, with park-hopping, and WP&more. But if I decided to pay cash for my 2 visits to PI, I would instead only have to pay for a 7-day MYW with park-hopping (but no WP&more). That would knock slightly more off the price of my MYW ticket than I'd spend on my 2 visits to PI.