Establishing things beforehand is all well and good for some instances, but making it a persistent theme just removes the point of characterization. Why would we need to roleplay a developing friendship/rivalry if it was already established thoroughly in the past?

Besides, it produces problems for the roleplayers themselves: If these two characters knew each other beforehand, then that means they should know personal things about one another that the roleplayers couldn't possibly, leading to just genuinely confusing and unrealistic portrayals.