Cabin bags are small individually, but they still consume working space together
It is easy to assume that five 20-inch suitcases should never be a problem. In reality, once several cabin cases are stacked together, orientation, layering and daily access order begin to matter. A boot that closes successfully on day one does not automatically mean the setup will remain easy for the rest of the trip.
First ask whether this is really a pure-suitcase group
If the group truly has only five standard cabin cases and very few loose items, many vehicles can handle it well. But real trips usually add soft bags, cameras, snacks, outer layers, water bottles and medicine pouches. Once those start spreading into footwells or the cabin, comfort drops quickly. The real purpose of an empty seat is often to give those high-frequency items a stable home.
Trips with frequent access need more margin than the luggage size suggests
If the group will repeatedly take out layers, medicine, child supplies, shoes or snacks, then simple fit is no longer the right standard. Every extra retrieval increases loading time and makes the arrangement easier to disturb. On those trips, a bit of spare space is often more useful than achieving a perfectly packed boot.
An empty seat helps preserve cabin order
Many travellers see an empty seat as waste. On a multi-day Xinjiang trip, it often acts as a buffer zone for jackets, day bags, temporary supplies and items that should not be buried at the bottom of the luggage pile. If that space prevents searching, squeezing and clutter around passengers' feet, it usually earns its place.