I play a lot of air, and one of the foibles of air-play is that typically you want the units coming out of your factory to do VERY different things depending on what they are.
Fighters should be sent straight to patrol, gunships rallied just behind the frontlines, and bombers rallied on the backline, and more variations of such. Unfortunately, currently each factory only has the ability to rally for all units. There IS an option in settings to only send fighters to patrol out of factory, and exclude all other units, but its a bit clunky as it clusters up all non-fighter units on the factory itself, and is simply less useful than building a second air factory most of the time, no matter how disadvantageous that tends to be in terms of metal costs. Furthermore, allowing for unit specific rally points could also present tactical advantages to ground players too. Allowing them to send tanks a bit forward, and AA and artillery a bit behined etc.