WP weapons are most commonly used for their smoke generating effect. They generate dense smoke that obscure vision that is also hot defeating thermal imaging from penetrating the screen.
Additionally, according to The National Institute for Occupational Safety and Health (NIOSH), the fire fighting information lists these methods:
*The agent may re-ignite after a fire is extinguished.
*For small fires, use cold water spray, wet sand, or wet earth.
*For large fires, use cold water spray or fog. Do not scatter spilled material with high-pressure cold water streams. Move containers from the fire area if it is possible to do so without risk to personnel.
*For fire involving tanks or car/trailer loads, fight the fire from maximum distance or use unmanned hose holders or monitor nozzles. Cool containers with flooding quantities of cold water until well after the fire is out. Withdraw immediately in case of rising sound from venting safety devices or discoloration of tanks. Always stay away from tanks engulfed in fire.
So the overall effect from their use are not as dire as you describe, but still not good if used against people.