If say for example Gollob is ruled out after his first ride, why not just say the remaining 3 riders take his rides, (rider must withdraw from meeting through Injury, not some fake headache or whatever). No need for an extra man in the pits.
I have often found this bizarre, if they want to keep the joker, it shouldn't have the effect of rendering the leading teams advantage gone in one race. Its about getting you back into contention, not all of sudden coming from nowhere to lead the meeting....