But what's the rationale behind one's desire to be able to do cost-efficient offense against fortified prepared enemy?
When I say cost efficient, I'm not suggesting a 1:1 relationship between defender IC costs and attacker IC costs. (Although some players no doubt mean it this way.) If Germany could win against the Soviets using 1:1 IC cost on tiles, that would be stupid.
But what I'm thinking of in terms of "cost effective" is the following:
1) The attacker can afford to put 3:1 or better firepower on a particular tile without the rest of the front collapsing to counter attacks.
2) If the attacker is using tanks, and the defender is using tank destroyers (or tanks, or SPART), the attacker and defender lose tanks in proportion to the number employed, terrain, entrenchment, disparity between stats on tanks, and application of air power.
What happens in some cases (due to how attrition works and other things) is that the space marines don't end up losing the number of TDs you might think they should be losing when their divisions take combat or attrition damage. Thus, TDs become more IC efficient through a quirk of mechanics than they really should be compared to the historical record.
I'm reminded of a particular battle for Leningrad that illustrates this handily. I'm the Soviets, and Germany is attacking Leningrad with a panzer army via Finland. I have several TD divisions that are basically space marines using 1941 HTDs. I say basically space marines, but they really aren't as awesome as some of the ones better players use.
There were several attempts to break the defense around Leningrad (including the climax of the defense of the city when Italy dropped a nuke on it in 1945). What's relevant for this discussion, though, is that the space marines defending Leningrad were basically getting 15:1 or better kill ratios on German medium tanks in battles despite Axis air power and solid leaders and doctrines. These kill ratios were only battlefield kills; when factoring in attrition, German losses were ridiculously higher. Note that I was using mostly 1941 TDs for a large portion of the war, while the Germans waltzed through Finland with Panthers.
Sure, a lot of infantry died in those battles, and yes, I did improve Soviet infantry weapons and gave Soviet infantry the anti-tank techs, but at the end of the day, Leningrad was a graveyard of thousands (tens of thousands?) of German tanks while the cost in Soviet tank destroyers was hilariously small.
Note: the defense of Leningrad wasn't just the urban tile, but the plains and forests in the area as well.
Now, if I were an arrogant man, I'd sit here and claim that it was my brilliant micro and strategic vision that gave the Red Army victory. But to be honest, before we tweaked some parts of land combat in our mod, the cost efficiency of TDs was a bit too good in some situations.