To be a Fire Observation Officer was something of great responsibility and required a person to be able to make calculations while under fire.
The dedicated FOO who was detached from an artillery battery had to adjust range and bearing for the guns if anything close to accuracy could be achieved. This had to be done from a datum point (his position).
If a FOO was displaced, he would have to give the fire mission from scratch from his new position, and that would add a delay, especially if the fire was coming from the same Battery.
That one spotter can't control the fire of several batteries at once probably mimicks the problems of radio-communications in WWII (i.e. the manportable ones at the time could not cater to several frequencies at once. In fact, most modern ones don't do this either).
Of course, I could be wrong. If anyone could give better facts to correct my assumptions, I'd be more than happy to take the criticism.