The proper listening angle according to ITU and many speaker manufacturers is 60-deg, resulting in a spacing between the speaker of 2/3 distance to the listening position (9 foot listening distance = 6-feet between speakers). Going wider than this frequently starts resulting in a weak center fill. It also shrinks the sweet spot.
For HT applications, having the speakers too far outside the screen edge will also pull voices and effects to the sides off the edge of the screen, reducing realism.
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