I can understand the goal of keeping tactical battles short.. In a game of this size hours long battles could really cause many to shy away... I will wait until we get a chance to test the tactical system before I commit any further to we need longer battles... for the record I have no problem with longer battles as I am one who will enjoy that as much as the adventure and civ building aspects of the game..
This, my friend, is why we can hit "Auto Resolve" ANY TIME WE WANT. You don't Have To Play Long Battles If You Don't Want To. That's fine. Just Don't take away the ability to play Long Battles for those of us (which I would say is the Majority of us) who WANT To Play Long Battles.
If you get bored half-way through a fight, hit Auto Resolve and be done with it. 
The player should be given the choice, period.
I fully agree though that a small battle, maybe 30 soldiers against a couple of monsters, would or should end quickly. I wouldn't even call something like that a "Battle" so much as I would call it a "Skirmish". I don't see how a Army of even 5,000 soldiers would be able to take a Entire City in 3 minutes unless it was a small city with No Walls and only Meager Defenses. It would take 3 minutes just to breach the Walls. Regardless of whether or not the battle continues during the next turn of the game, large battles should take more then 3 minutes. 5,000 men wouldn't even be able to run the length of a football field in 3 minutes, much less breach a defended city and kill everyone inside (unless you use some seriously destructive spells).
Speaking of Spells in battle. I Do Fully See The Possibility of even a Large Battle being over rather quickly if a Sovereign or other Powerful Caster Type of unit was present at the battle. Or maybe even a Dragon. I can see a Dragon killing a few Thousand soldiers in only a few minutes, sure. For some reason though I Can't See how 10,000 Mundane Soldiers vs another 10,000 Mundane Soldiers would end so quickly, unless the battle mechanics are seriously gimped.
I'm fully willing to "wait and see" and "try it for my-self" before I pass judgment though, How-ever by then it Will Be Too Late for Us Beta-Testers to have ANY SAY What-So-Ever in them being changed at this late stage of development before release.
Like I said in another thread the other day, if this isn't Done Right the First Time then Elemental will get Bad Reviews and it will be tanked by gaming sites before it ever really has a chance to get off the ground. No Matter What I DON'T want that to happen and I doubt anyone else here does either. We ALL want Elemental to succeed and be the next Successor to the Legend that is MoM. Even Age of Wonders, though it was a Great Game, didn't quite capture that Magical Feeling that games like the original MoM, MoO, and X-Com had.