Splash guards are molded and provide minor coverage. True mud flaps are heavier, wider, longer, and stop much more debris. Additional receiver mounted flaps like Rock Tamers provide extra protection for trailer loads. Each has a purpose.
Depending on rear overhang and trailer tongue length, you may not need additional coverage but receiver mounted flaps will definitely help keep debris from your trailer.
I have DuraFlaps on my current truck and love the additional coverage. My trailer has drive on/off ramps on the front that protect my trailer loads from most everything else.