AVR regulation brushless vs. brush alternator
A brush alternator, with a static exciter, uses brushes to directly connect the rotor coils. This is expensive, it needs a lot of current and brushes that need regular replacing. The control electronics are big and expensive.

The brushless alternator uses an additional stator coil magnetized by the AVR called Field. A rotating exciter coil, followed by rectifier diodes on the rotating part generates the main rotor current.  This is cheap, since you need little current on the Field to generate a high current on the rotor. No wear on brushes, and cheap electronics.

See this image from this site:
  

a) is only used on very large machines with very high excitation currents. Like power plants.
b) a brush alternator.
c) a brushless alternator.  



The advantages and disadvantages change when you're looking at tiny sub 1kVa generators, like in your car. Those are a different class of machines, more like a dynamo. It's cheaper to replace the entire unit instead of only the AVR.