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Don't forget about gas mileage in the compromise equation. I think the two biggest variables when it comes to this are performance and gas milage. The cost difference between making a 9.5" wide wheel vs. a 10" wide wheel is negligible and anti-hydroplaning characteristics falls under performance (but is more of a tire trait than wheel width). Same with comfort.
The more performance-oriented the toal package, the higher the cost and upkeep (see Ferrari, Porsche, etc.), and the lesser the waranty. I think a better question is why did BMW go with 1/2" widths at all (most brands go with widths on the whole number). I think BMW could have went with 18x9 and 18x10, 255/35 and 275/35 with very little ill-affect on gas mileage and comfort, and not much more cost.
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