I'll back up Todd on this, having ridden almost every int. open sportbike race in the last few years, the volunteers heading out to the corners is a good indication that it's about 10-15 minutes before it's time to go out on track.
That said, I think it's a reasonable request to have a 10 minute horn as every other race start you can judge by how far through the previous race it is, so it can add a bit of stress trying to figure out how much time is left. If the horn was given when the volunteers were being driven out to the corners, then nobody would have to come back early from lunch to give a warning.