Raubahn has been sitting in queue for an hour and is now next to login
Pipin was in PotD and DC'd and has just logged back.
Van Baelsar was AFK in a quest hub for an hour and got bumped, and has just logged back.

- who should get the one spot they have available?

The proper answer is to cap logins at 98% of capacity and use the 2% as buffer room to resolve this, while also not freeing a spot for 5 minutes after any "abnormal logout", for the most recent 1% of capacity of such abnormals.

- But if you don't have this option... somebody has to go into the queue.

Do you really want to explain to Raubahn why he was at position 1 and has now been bumped to 2?