The majority of people coming into Brighton in their car would do so via the A23. It therefore makes far more sense to use a site on the A23 for Park & Ride. Sending them to Falmer (a detour of a good couple of miles) doesn't really represent joined-up thinking to me.
When you say 'proper Park...
This is true. I was on one of those trains.
However, if memory serves me well, you could only alight from the front six carriages at Moulsecoomb. You could get out from all eight at Lewes.