does anyone feel like this penalty is actually a good thing? Any arguments FOR it? 'Cause right now I am leaning towards open ground being the traditional no modifier.
I am in favor. For reasons of realism and gameplay both. Granted, it makes things more complicated.
I think calvary doing better in open ground is the only real reason for the defensive penalty there, and they could be given that bonus separately. In real life, most major battles were fought in open ground. If it were that hard to defend, they wouldn't have done it.
I disagree that it is unrealistic. Combat favors both attacker and defender depending on the situation. Generally the offensive side is bound to come out on top in a war of equal strength assuming they don't go off picking poor fights.
Attacking is having the initiative. The element of surprise. Choosing when and where the battle will take place. Need to be able to see the enemy and to move without restraint for this. In open terrain against a completely unprepared enemy this is the case in Civ5. This is the -33% for defenders in open terrain.
If they are not in open terrain unprepared, the bonus for attacker is lost. Makes sense since hills and forests are naturally easy to use for concealment and cover, while slowing down the attacker. Don't need time to prepare for it.
While in the open, then, if the defenders are prepared? That's the 50% fortification bonus. Defenders stronger.
As counterintuitive it seems open ground penalty is actually feature that makes defending better than attacking.
That as well. As you said though, the combat is a lot more complicated than Civ4. It turned tactical. Knowing the force mix and amount required is just part of it compared to 5 with new modifiers, logistical issues and ranged combat.
You're assuming the defender even has a choice.
What if you have to defend a big open plain?
You can do either of these three (including all three at once):
1. Fortify. +50% (a few turns of foresight required)
2. (no foresight required) Attack them as they approach within range (this is known as a 'spoiling attack') as suggested.
3. Build defensive structures (fortress or citadel) (foresight required).
I don't even see a game justification for making defense so weak most of the time and so conditional on terrain. It's not even a question of tactics per se: the foolish no-stacking approach means that units can end up shoved out into the open because the Amazon jungle isn't large enough to hold all of them.
Defense is not weaker. There's fortification bonus, forts and/or choosing to attack first. Defender is covered.
The combat isn't tactical because it is possible to fill jungles with units completely? It does not follow logically.