
I have really no idea if we play the same game, due to the way active defense works (siege, the movement advantage inside your borders, fast mounted flankers, practically no healing for the incoming units while they travel several tiles inside you border, nationhood and/or slavery civic) I can frequently wipe out AI stacks that 1:2 or even 1:3 (in the long run, even higher kill ratios are achieved) outnumber me initially (on immortal) - as said good reconassance is necessary.
On deity, in game-phases where you are not geared up for war, diplo- and espionage manipuation is way stronger than trying to match huge stacks.
I'm really not so sure either at this point. 1:3 is a *longshot* with active defense in my experience assuming tech parity (something that can be another longshot on high difficulties). 1:2 is doable, but also questionable especially based on terrain - if your units lose fights, you don't do flanking damage... If the incoming stack is large enough, you need *lot* of siege to churn through all of those units and get it to the point where your knights (probably outnumbered 3 to 1, since a lot of your resources went into producing siege rather than straight up unit killers) are going to be winning/escaping a good numbers of fights. And why am I talking about knights? You'll likely be dealing with a horse archer VS pikemen scenario if you're playing on high difficulties And against a larger stack, there's a good chance you're going to lose a good number of fights as the CPU hops from hill to hill as it often does. Also, those flanking promotions that you're giving knights take away from other promotions you could be getting thus lowering base combat odds, meaning you may be fighting some *very* nasty anti-horse upgraded pikemen which just dwarf your horse/knight odds. And heaven forbid you have a really culturally contested border and don't have room to work all of your hit and run action as they move in on your first few cities.
And that's assuming you even have the ability to flank incoming units. If you've got a handy-dandy stack of knights when the opponent nabs steel, go figure - you can't flank those cannons and your defense becomes a faint prayer for cuirassiers.
Add to this... Small empire with no iron? Tough luck - no knights for you. No horses and you're just hooped from the get-go. What's the contingency plan? Restart and pray for resources? The early part of the game is when it's really tough to secure resources on higher difficulties, and active defenses requires not one but two resources to keep you afloat in the earlyish game.
Protective super units... I probably don't need to slavery - I don't need to keep around a bunch of siege to soften them up AND a bunch of knights to kill them after, so my force will be smaller, and each unit is cheaper. If I do need it, I'll need it less for the cheaper longbows. I do find it ironic that a lot of people worry about pillaging their land with a passive defense, while slavery popping a good portion of their force in a pinch is part of their defense plan with the active defense... With protective, I don't have to worry about resources. I don't have to worry about having enough land to do multiple hits before he reaches my city. I don't need to worry about what terrain they are sitting on (which can be a bummer if you have a lot of hills in your land, and kept forests become a *massive* liability). And even against higher tech opponents... Protective longbows and muskets can hold their own quite effectively (on the defense) against grenadiers and rifles, and pre-cannon siege has to work incredibly hard to do significant damage against them.
So many more hypothetical negative factors plague the active defense than the protective super-unit wall, I think *relying* on a 1:3 kill ratio for an active defense is wildly optimistic. With protective "passive" defense, the only factors that really matter are my upgrades and the terrain I'm on. I can control those a heck of a lot more than the factors weighing against the active knight/siege defense.
I've tried active D quite a few times, and it just seems more costly and less reliable.
Oh, and, just because I'm only talking about pro from a defensive perspective doesn't mean I don't find it offensively viable as well... Actually, I think it's pretty spectacular on offense.