While I get the want for realism (believe me, I do), in practice I doubt it'll ever be feasible.
In an invasion thread, you've got anywhere from 20 to 60 people posting with considerable frequency. Even if you take into account that not all of them are going to be on the same objective, you're still left with a sizable number of people fighting each other.
Every single instance of NPC combat that I've seen done in invasions has been an utter, complete mess. It's breeding grounds for all of those nice things that [member="Rusty"] listed out and I won't bother repeating. Having been involved in judging the outcome of four such invasions (Wayland, Ord Mirit, Contruum, Balmorra), I can say with no small amount of certainty that it was always the NPC objective that was most trouble to judge. When you've got such a huge number of people trying to control troops, machinery, terrain, defences, and Force knows what else, the whole thing is reduced to a mess not worth bothering with.
Fleeting isn't really my territory, but I've seen examples of it proposed and executed with relatively little trouble as compared to NPC combat.
What I'd like to stress here is that the more aspects of a situation opponents try to control at the same time, the crazier it gets. There's a reason many people prefer the uncomplicated task of writing the impact of conflict on a single character. Not to mention that in duels, the peen-measuring contests of who has the baddest tech are relatively limited, whereas fleeting and NPC combat usually see it get out of hand in frustrating, if impressive ways.
That being said, I still have good fun with most invasions. Manaan was hands down the most enjoyable, though looking back it was probably far less so for the people running it.
I'd love to see more story and plot hooks in invasions, and less stats-y and objective-based play. Also, permanently relegating NPCs to background noise would probably save some future FAs a world of headache.