What are the adverse consequences of driving around with a dirty air filter?

Or, put another way, what are the benefits of changing a dirty air filter?
What are the adverse consequences of driving around with a dirty air filter?
A clean air filter is important for your car's performance. This means that your air filter should be changed regularly, or your gas mileage will suffer. A dirty air filter can also cause other systems to run ineffectively, such as your emissions control system, which regulates the car's air-fuel mixture. A dirty air filter can also cause your spark plugs, which ignite the fuel in the combustion chambers, to foul, as there will be too much fuel without enough air; if your spark plugs foul, you may have problems keeping your car running.
What are the adverse consequences of driving around with a dirty air filter?
Gas mileage gets quite a bit better if you have a clean filter.
Fuel injection in modern engines will compensate for a dirty filter. The most noticeable result will be a drop in horsepower as the in injection system compensates for less air flow by reducing the fuel going into the engine.