From an OH&S perspective alone, might be a case of better the devil you know Kev. Given that wheel contact should be an accident, I'd rather the status quo with pneumatic tyres. A horse clipping a solid tyre might be more dangerous to that horse and horses following. The stewards will always investigate and handle the incident appropriately, minimising repeat offences, which may be missed if solids are used.
Whilst giving the punter a better run for their money is always a good thing, I guess it's like a galloper clipping heels or a greyhound being galloped on.