An extra suitcase is often less useful than it seems before departure
Before the trip, travellers tend to imagine that every possible item deserves its own space. In practice, the most frequently used items on a Xinjiang road trip are usually outer layers, water, tissues, medicine, chargers and child essentials. If those things end up buried behind tightly packed hard luggage, the group does not gain much comfort from carrying one extra case.
A spare seat creates daily value, not just visual spaciousness
An empty seat is useful in more ways than people expect. A senior can stretch more. A child can rest better. Bags, jackets or camera gear can be moved without crowding feet and laps. If someone feels carsick or wants a calmer position for part of the route, the spare seat becomes an immediate pressure-release tool. That flexibility can improve every day of the trip.
The better fix is often to edit the baggage plan first
If the only way to carry everything is to sacrifice the last comfortable seat, the packing plan may still have room to improve. Separate infrequently used hotel items from daytime essentials. Use a soft bag for high-frequency items. Reduce duplicate shoes or bulky just-in-case gear. Better baggage layering often protects both comfort and carrying capacity.
The exception is when the extra luggage carries genuinely non-negotiable gear
There are cases where the extra suitcase deserves priority, such as infant gear, medical support items, or specialised photography equipment that cannot be reduced. The real question is whether that extra case carries a true requirement or only extra uncertainty. If it is mostly precautionary packing, the human comfort value of the spare seat usually wins.