When looking at fan duty tradeoffs (well, post-warm-up - obviously we want the fans off until the car is basically warmed up), as far as I can tell there's only two downsides to running the fans full speed all the time: Current draw on the alternator adding drag to the engine, and wearing out the fans faster. I could care less if the fans fail every 3 years instead every 5 years or whatever, cost of doing business. I would expect the current draw to be offset by the power gains from running cooler by a mile.
Then there's MPH thing, but if you've got other devices stacked up with your radiator for direct flow (oil coolers, intercoolers), then even at speed I think the fans are helping move air through the stack better. So my philosophy on the whole thing tends to come out to "ignore speed and AC stuff. If the coolant temps are 180+, fans go 100%"
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