From the web site:
Parking:
Nearly 6,700 spaces are available for vehicles. Spaces are available to cars and motorcycles for $10.00 - recreational vehicles for $12.00. Preferred parking available for $15.00. Platinum Passport members receive complimentary preferred parking. Gold and Silver Passport members can park in general lots for free, and pay only $7.00 for preferred parking.
If you pre-pay you don't save much {well, 61 cents} 'cuz there's no special pre-pay line (not last time I was there).....so you sit in the parking line and save the 60 seconds it would take them to take your $ and give you the ticket...... 'cept it takes that long to scan the bar code of your pre-paid parking.
So the only advantage...2 advantages are: save less than half a buck and another thing 'pre-paid' .