I think the fly being cut lower, so that you can pin it to the ground is one reason.
6.5x284 really has a solid example to consider, especially when you consider pitching the shelter off the ground for increased airflow - sometimes, especailly in late Sep/Oct here in MT, we get some really wet days from either extended rain or snow. That level of humidity is something we just do not see a ton of, and it is super easy to overwhelm a shelter. Smaller 1p tents most especially. The comment that a hybrid/single-wall DCF shelter may not work well for higher mountain hunts, especially extended outings, is a huge consideration. If you've got a stove to dry the shelter out that's different - but when you have a high condensation night, having space, and an inner tent really makes life a lot more pleasant. You can always start a fire (in most places) to help dry your clothing system, but shelter and sleep system is another thing. A lesson I seem to encounter every couple years but do not always heed.