I thought that a car had to be weighted to enable it to run? I remember reading in John Wardley's book about a car stalling because it didn't have sand bags in (not The Smiler, I forget which one it was) Plus, I live in Stoke and it's been really windy here today, so I doubt this would have helped an un-weighted car.
They can run without sand/water containers, when the Smiler was first built there was a lot of adjustment and wheel changes to make the ride work, planning and space restricted this ride so much, it is on the edge of physics when it comes to getting those cars around the track, conditions need to be close to perfect, which is why in my opinion, they were sending empty cars around.