I suppose the two arguments could be.

In Favor of Access: It helps with the transition of power, and continued flow of governance, it allows former Presidents to encourage the current president to make good choices, and also helps the party formulate a long term strategy when they presumably regain power.

Opposed to Access: Former Presidents are now civilians, and allowing individuals outside of the current administration to Access sensitive information poses a national security weakness, as malicious individuals could use that information to undermine the current administration, or are a weakness if exploited due to potentially weaker security in comparison to active administration individuals.