System-wide load factors are in the low 80% range. There are still a substantial number of unsold seats to accommodate last minute walk-up sales, awards, employees, and irregular operations.
Well some airlines are run better than others. All I can say is it's been a long time since I've seen more than a handful of empty seats on a plane.
Total load domestic factor for 2014 was 84.58%. The peak was July with a non seasonally adjusted load factor of 87.78%.
As you point out there are a number of reasons airlines might want to keep those seats available so they can profit from walk-up sales and deal with irrops effectively. I don't know exactly what point flights are "full" for the purposes of saying awards are necessarily forgoing revenue but observing airline behaviour with restricting availability and devalueing programmes I suspect we're reaching it.
Keep in mind that prior to 2001 total load factors were around 70% and off-peak loads dipped to around 60-65% regularly. That was the environment the FF programmes we enjoy today were crafted in.