Remember that most non-revs are NOT positive space. Instead, they are space-available non-revs, meaning that they get on the flights only if space is available AFTER all revenue passengers have been accommodated.

Examples:

1) D2 employee travel and D3 buddy passes. Basically, the standard non-rev situation in which an employee wants to travel somewhere for minimal cost on AA/AE/AX. He or she must get in line behind ALL revenue passengers, including those on award tickets. It often comes down to gate control at the last minute, and at that point the employee could end up with either row 47 or row 1 in 3-class F, depending on what is available.
(Some AA employees I know dislike playing the standby game so much that they actually buy revenue tickets instead. Some even earn status! And I know one FA who earned 2MM and Lifetime PLT! He says whenever he calls Reservations they get very confused because they don't know whether to process his PNRs as a PLT or as an employee!)

2) Pilots/FAs/other AA/AE employees commuting to and from their base. These employees are completely on their own to get to work on time, and must be on the standby list BEHIND all revenue passengers. Nobody forced them to live in SFO if they are based at JFK, but many international FAs put themselves in exactly that situation. If they don't get on the feeder flight and miss working their flight out of the base, they could face repercussions beyond just not getting paid. Hence why flights such as the early-morning RDU-JFK and DCA-JFK flights are often called the "crew shuttles." They are operated with ERJ-145 (larger aircraft than needed at that time of day), some believe in order to be sure that all of the FAs and pilots can get to work on time.