We used to clean dozens of carpets for Newcastle Breweries. They only had them cleaned when the environmental health officer told them to.
This was usually because of the black build up near the bar hatch. This is beer and dirt and can be removed, just takes a little effort. Enzyme pre-spray, do the less dirty parts of the carpet, return to the black parts, may need to do this a few times. You sometimes need to mix the pre-spray with really hot water to break the deposits down. These were all done with portables and airmovers to dry off. Truck mount would have done them in half the time.