Read up on the studies yourself - the 40 hour work week was established based on output studies of factory workers - people who, like you say, aren't doing anything complex. They still suffer from performance penalties just like everyone else, and they also eventually go into negative output mode, just like programmers.
Having done the grind work while strapped for cash, I can tell you that the allure of money and the desperation of making rent will only make you productive for so long. Everyone has different limits, but at the end of the day someone who is overworked will likely give up large sums of money just to work less.
As an employer, why can't you give your guys normal 8 hour days? It's more economical for everyone - you are getting sustainable, high output, as opposed to the illusion of working hard, while getting very little out of your employees in reality.
But then again, I don't run a successful consulting firm.
So, even assuming that simple work will allow you to get a significant long-term productivity boost working people 8 hours instead of 12, in the long run, you're going to be working with a set of people who'll likely be challenged by the work, so fatigue will cause problems if they're challenged at 8 hours/day, or you could be getting smarter work out of them if they challenged at 12 hours/day.
Of course, building the tools to automate drudgery does take some time, and considering Ars Digita apparently operated during the first .com boom, time-to-market was critical, so some of this logic doesn't apply to the original example.