Exactly right. The main intent of HT speaker positioning is to minimize the impact of room acoustics, as you put it "to make the listening area more in the direct field than the reverberant field." And, as you mentioned, to tightly focus the sweet spot in a specific listening area. Ideally, to suck the listener into becoming totally absorbed in the story almost as a participant.
By the same token, many audiophiles complain that THX certified speakers sound "lifeless" when reproducing music, because they are deliberately designed not to colour the tones in an attempt to closely replicate the original soundtrack.
When listening to music on the other hand, the intent is often to expand the sound field, even to deliberately introduce ambiance effects through reverberation/delay (DSP, for example). Also, you usually want to expand the sweet spot as much as practical, because people often don't stay in one spot during music listening the way they do when watching a movie.
Additionally, many music speakers are designed to produce an artificially "warm" coloured tone, to overcome the perceptions of those who believe that digital music and/or solid-state amplifiers "cool" the original sounds. And, rather than being a participant, the intent is more often to induce the feeling that the listener is in the audience at a concert hall.
So there are a number of reasons someone might want both "A" and "B" main speakers, in different locations. And if you REALLY want to go to extremes, check out some of the top Denon receivers. They even have "B" switches for the rear surround speakers, so you can position a set of REAR music speakers differently from your HT surrounds... Grab one of their brochures for some interesting diagrams of how they recommend you position your "A" (HT) and "B" (music) speakers in both front and rear. Let's see: 7.1 for HT, plus four more for music -- my spouse would not be amused :)
