I've thought about this a lot recently, because I win a lot yet I actively choose to play reactively and its fair to say im not the worlds greatest deployed.
That said, list building is still a massive part of the game even for me- its just im building for capability, not specialisation.
This also impacts on how you can play the game- modern style msu armies have lots if small specialised units that really can only do one thing, taking the tactics out of the game. Multi-capable units, on the other hand, can fulfil mltiple roles in the game and deciding what to do and when, gives you a great opportunity to frustrate the more obvious behaviour of opposing specialised units.
Oddly, the current Deathwing build (that is by far the best list for them) kind of lacks flexibility- every unit has very specialised ways to play. Doesn't really impact upon their effectiveness but an interesting aside.