Park-and-rides are not the answer because of the Law of Mode Inertia: people in cars tend to stay in cars. Parking lots and garages will not get people out of their cars, at least at the parking end of the line, and thus they will do nothing to revitalize walkable neighborhoods or build a constituency for walking and local transit. So why do certain transit planners seem to love putting park-and-rides in their projects? Why have they shown up along the Northern Branch of the Erie Railroad and the North Shore segment of the Staten Island Railroad and the "transit-oriented development" for the Tappan Zee Bridge? Why was so much stimulus money spent on a huge garage at the Botanical Garden?
There are a few reasons that come to mind. The first is a fear of empty trains. When you introduce a new product, it rarely catches on right away. There is almost always some adjustment period while people find out about the new product, try it out and change their routines. The sensible thing is to have enough money saved up so that you can keep providing the service until it catches on.
Public transit planners don't often have that luxury. Government budget hawks are ready to swoop down a the sight of Empty Trains, and the climate makes it hard to ask for operating assistance. They want the thing to be successful from day one, which means catering to everyone who might conceivably want to take that train, including some people who live more than a short walk away and don't want to take a bus.
The others have to do with goals. For many years, planners didn't appreciate walkable neighborhoods, and if they do now they don't recognize the value of transit in delivering the pedestrians who would walk in those neighborhoods. This relates to the Law of Mode Inertia: somehow they haven't figure out that every commuter who gets off the train and drives home is one less shopper at the walkable stores, and one less diner at the walkable restaurants.
These planners also don't seem to understand the self-identification correlate of the Law of Transportation Inertia, and its role in the transportation cycle: park-and-riders tend to identify as drivers, so that every park-and-rider is another driver who's going to fight for subsidies to drivers at the expense of transit.
One of the big goals they do have, however, is to "eliminate congestion," and park-and-rides are very good for that.