Hmm. Maybe I don't fully understand this system but from what I gather, it still seems pretty convoluted to me; even if it implemented that spot number thing.
I'm from Singapore. Here, every vehicle has an electronic device with a stored value card in it. On the way in, a wireless scanner logs you in, and on the way out the same system clocks you out, calculates the total time you spent in the lot (minus 5 minutes goodwill to allow you to find a spot) and deducts the appropriate amount from the card.
Both the entrance and exit are gated and the gates won't open if 1) all the lots are full or 2) you don't have enough value in your card to pay. Top up stations exist within walking distance so you never end up being unable to leave.