Why did you have to replace the evaporator and pressure switch in the first place? Evaporators can last quite long.
Regarding the air filter on the evaporator, it simply prevents the evaporator from accumulation of dust. It is sort of like the air filter on an air conditioner. You can still run the AC without it but it will clog up the evaporator sooner and will require the AC to be serviced sooner. With the filter you can simply replace the filter without having to open the evaporator. Some cars do have it while others don't. But if there is a place for it, then you should probably use it.