Since the user is already logged in, this is pointless.

I am not sure it is true. If the user does not interacts with loginwiki in some way, The global (loginwiki) session may be expired earlier than local wiki session (imagine you have been visiting Wikipedia for a week but does not interact with loginwiki in any way, then the browser may expire your loginwiki session though keeping the Wikipedia session). So in this case doing central login would be useful.

Note this circumstance may already happen currently - we may want to measure how many logged-in user does not currently have a valid global session.

This is a good reminder that we will have to think about how security reauthentication will work (whether or not we want to change this specific aspect) when we work on T348388: Use central login wiki for login (SUL3).

In T356614#9512394, @Bugreporter wrote:

If the user does not interacts with loginwiki in some way, The global (loginwiki) session may be expired earlier than local wiki session (imagine you have been visiting Wikipedia for a week but does not interact with loginwiki in any way, then the browser may expire your loginwiki session though keeping the Wikipedia session). So in this case doing central login would be useful.

For users who check "keep logged in", both the central and the local session lasts for a year. Having to log in once a year is fine. Users who don't check the checkbox are on their own (IIRC their central session expires after a day). In any case reauthentication is too rare and erratic to be worth thinking of as a session keepalive mechanism.