For me the defender advantage must be only the good positioning and/or the support of the static defence but in any case it should be the stupidity of the attacker shooting automaticaly at the wrong target while you can shoot it like a duck.
Imagine the following breifing
"Let's place couple of tent there, there and there, Why ? Because they can't stand the sight of a tent, they'll go berserk and empty their gun on it, while they do that, we might have a shot at them. In the same idea we maybe should build a ref during the attack for no reason, they are so dumb they could get scared and try to destroy it instead of shooting at us, and it's even better if we have someone to drive toward them in a civil ore truck, they hate those too, might give us more time. .... Oh yeah and if we have time we can place some pillboxes, sometime it helps too, but tents first !"
-Lt MrStrategic, in charge of perimetre defence-
I'd rather have to defend a front with some defence well placed, couple of jammer, and back artie, and work the positioning until i can see something that can hold the attacker for a reasonable price basing the fact they will ignore all non offenssive structure and strictly shoot the defenders, that would start to be a challenge or manage my game to avoid at all cost to have enemy in range of my base.
As for the combining T1/T2/T3, Ofc they are amazing togather, few unit well chosen, well mixed but more than everything well managed by placement, micro and timing can take over an army twice as big and with minimal losses. That's the only reason i have been trying to do some "tech-rush" build with both allies and soviet. Soviet have the IC "flak/apc" with V2 and infantry support and allies have tanya/jammer with armor and infantry wich allows a blob to be impenetrable from both infantry/vehicule. So far my success only depend on te skill on my opponents, means only the skill define the effectiveness of this build, means I need more training to be able to tackle better players than me. I can see every game i lose with that build by the replay that the mistake is my own, not to blame on the build and/or the opponent. Sometimes i can kill for 8000 to 10000 without significant loss and sometime when the jammer get berserk on it's own, the armors are camping behind the infantry, tanya for no reason stand few cell alone from the blob just to get sniped, i can lose everything without inflicting any damage. So ... yeah it's freaking hard.
In a game defending with only 2 pillboxes, a turret, a jammer(behind a peice of concrete), 5 rockets and 2 artie total : +-5000 defense, i killed for 10000 and forced my opponent to retreat and the pillboxes were frontline so no barrack to tank or anything, just good positioning/micro of the static/jammer/artie and few rocket soldier. (My army was elsewhere pushing and i was expeting this counterpush).
I think the behaviour can unlock more strategic potential than just "Hey dude where is the skill if your unit are smart, it's too easy !".