Not so on A320 variants. FCOM2 states actual landing distance does not consider the use of reverse thrust . It then lists contaminated R/W states vs landing distance.
This distance may be decreased by a factor if reversers are operative, but they are not essential.
HOWEVER they are required for contaminated R/W take off, to be used in the engine out decel phase following a rejected takeoff.
So yes they are essential but only for contaminated R/W take off.
Fuddy Duddy