Air dryers add significant cost and maintenance but provide little protection against corrosion.
Dryers only reduce the amount of new water that is introduced into the dry or pre-action piping. Unfortunately, most dry or pre-action systems already have plenty of trapped water in the piping from the initial hydrostatic test and from periodic testing of the system to meet code requirements.
Compressors without attached dryers add warm, moist oxygen each time they turn on. Compressors with attached dryers simply add warm, dry oxygen to the system piping. With regard to corrosion in dry systems, adding fresh oxygen is far more damaging to the piping than adding a small amount of condensate water. Air dryers add significant cost and maintenance but provide little protection against corrosion.