Review the photo:  

4 plow trucks/one equipped with TowPlow.  Total swath for each pass by all 4 trucks = 50 feet.

4 plow trucks/if all were equipped with wing plows:
Total swath for each pass by all 4 trucks = 56 feet.

Total Costs:  

1 Towplow:  $100,000.00+
4 Wingplows:  $40,000.00

Towplow versus WingPlows:

TowPlow requires dedicated truck; if it goes down the Towplow cannot be utilized.
Towplow can only be used on multi-lane roadway.
Towplow equipped with liquid or granular distribution system requires a salt distribution control system capable of operating two different hoppers simultaneously.

If all 4 trucks are equipped with wing plow, one truck out of service results in loss of ability to plow 14' swath only.  You retain ability to plow 28' swath with other two trucks.
Wing plow equipped trucks can be used on any type of roadway:  single lane or multi-lane.
No special salt distribution control system required.
Trucks with wing plows can cover separate routes when not gang plowing.