The reason for spotters and snipers trading off is usually from what I have read is from fatigue setting in fast. You can't stay mentally or physcally alert for long periods of time when nothing is happening, and they have to take turns getting some shut eye too. From what I remember the spotter usually carried the radio and a M203. The sniper had the M40A1. As a rule in the infantry the junior guys always get the job to carry the heavier or extra shit(hello radio).
I felt sorry for them when they had to carry the SASR and the ammo for it.