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You are part on the mark, with a multi speed transmission the only relevant ratios are the overall in the lowest gear and the overall in the highest. You are again correct about having adequate gradability, but it is not as trivial as you make it out to be. While a lower (numerical) gear may successfully pull away from such a grade it may generate excessive heat while riding the torque converter (or clutch slippage). Also many do not think of the accelleration advantage gearing does give you until you do real world comparisons (just try starting in second gear with a manual to illustrate).
The other factor is the high gear, I know of no OEM transmission manufacturer (including HD truck) that offer a significant spectrum of gear ratios, so to get the appropriate startability you have to live with some measure of comprimise on high gear. Also when pulling a heavy load you do need more RPM in high gear to generate additional power.
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