Originally Posted by
levitation
It also means that you will have plenty of empty seats on your flight and you can move around if you do not like the seat you are in, or at least have the seat next to you empty!

This might or might not be the case. An airline can still show D4 C4 even when there is no spare seat left. It is also possible that upgrades will fill the cabin so there might not be a free seats.
Booking Class availability is more an indication of how full the cabin is rather than a reliable tool.