on a road car, there's a danger that they'll:
- be very noisy (you might sound like a S1 Landrover, whiring down the road)
- wear extremely quickly; typical road tyre tread wear rating might be 300 whereas typical track tyre might be 80
- be dangerous in standing water
- cost twice as much as a good road tyre
- have less grip than a good road tyre unless you can manage to get them up to working temperature (which you just won't on the road)
- will break away with much less warning (track tyres aren't as progressive as road tyres)