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You have to consider the torque loads of every component in all situations.
Overdrive trannies allow you higher gearing with lower torque loads on the driveshaft, tranny housing, output shaft and related parts.
Imagine a 100 ftlb engine with 3:1 1st gear, .75:1 OD, and 4:1 final drive. 3:1 overall in OD, 12:1 overall in 1st means you get 1200 ftlbs at the wheels but only 300 at the tranny. Change that to 4:1 1st, 1:1 OD, and 3:1 final drive and you still get 1200 ftlbs at the wheels and 3:1 OD, but you now see 400 ftlbs of torque at the tranny, driveshaft, etc. and therefore need to be larger and more heavy duty.
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