[Hadrian]

Since I only own one army, my opponent always knows he's taking on the Dark Angels. Granted, he may butt heads with the Ravenwing or the Deathwing (someday).
Regardless, my opponents are going to optimize their army to take out Space Marines, so I try to optimize (NOT min/max) my list for his army.

Since almost every army (especially since 4th ed.) allows plenty of variabillity in tactics and composition, I don't think this skews the fairness of the game at all.

OT: To avoid being part of the calculator squad, I keep my lists. Sometimes I make changes or little notes about what worked and what didn't. Now I have a broad selection of pre-made lists that cover a nice range of points and tactical optimizations.

Now...about all this comparison to real-world military planning...you've got to keep in mind one thing:

This type of argument can justify both sides of the discussion.

Given enough time and resources, in the Strategic and Operational levels of warfare, opponents can adapt to each other. You might say that the one who adapts fastest has the strongest advantage.

But at the tactial level, small units, and small spans of time, are far more limiting in their ability to adapt. Squads, platoons, and companies, have specific training on very specific equipment. One could argue, at this level of warfare, that successful commanders can apply what they already have to maximum effect.

So knowing this, rather than coming down solidly on one side of the discussion or the other, you can vary your playing style (Especially in campaign games) to simulate these kinds of developments.