I wish we could put a limit on it somehow, but I think a better way would be to set the control tyre as something that will last all day easily. Didn't schumacher used to do a compound harder than blue (red?), that way if they didn't wear there'd be no reason for everyone to keep throwing new tyres on.
The only time I think people would really struggle for grip would be a worn grass track, but on the other hand that might make that kind of track more interesting to race on
I do get a bit off my tyres but it's still the biggest expense for me at nationals, on average I'll use more than 80 pairs over a season (assuming it's all dry, rain my increase that number), which at retail is approaching £600 - and thats too much IMO.
The other issue we have thats making people use more is how competitive the series is, don't get me wrong this is a fantastic thing and makes a good result even better. The top 20 or so are now all so close everyone is looking for that last little bit of performance and an easy way to get that a lot of the time is new rubber, a couple of seconds over a couple of runs can make the difference between back of the A or top of the B, and you can't win the meeting from the B...