That's a good point LAX. I must admit that I was under the impression that if you had not pre-selected your seat, it had already been allocated to you when you checked in, but even if that were the case I don't know at what stage it is done
Most airlines now just allocate as you turn up. In the days of automated kiosk check ins, you can still choose your seat at the airport, so, empty seats on seat plans may not be empty.
Been on plenty of flights that show half emtpy planes according to the seat plan only to go out almost 100% full when your on them.