I've been curious about this too, and realize its all up to operations management.

A year ago there were more than 10 of us that missed catching ORD-BRU flight... we all saw the plane still at the gate, but they weren't letting any of us in. Many of us were of status (EXP, Plat).

I couldn't believe they would not wait for so many of us, and for so many frequent flyers.

However, I was put on a ORD-LHR flight 2 hours later, and it was only 1/2 full... so they could easily accommodate European flyers since AA flys to London often. So the inconvenience was not being on the direct flight and landing in my final destination a few hours later... but AA only has to get me to my destination, regardless of the time.

So, now I would no longer be surprised if I was cut off at the gate due to a late connection, and still see the plane at the gate but wave bye to it...