As per my findings, this should not deter your clients from getting the .gem updates.
However, I am curious, why would you NOT want the new scan engine ? When something new is coming up, its definitely for better ( Except VISTA ) and it is bound to have better impact on the resources. Also, any given scan engine undergoes a decent beta life before it hits the stands real time.
So if I were you, I would just let the update happen.
See McAfee KnowledgeBase article KB52426 for an explanation on why Engine updates are necessary from time to time.