I agree, the ability to create a perfect 'take 6' digitally is really neat, and would be a great tool for films on shoestring budgets who can't afford to waste film, but I can think of at least three reasons/counterexamples to say that more traditional techniques are better.
One, Fargo was shot on a shoestring budget, and they couldn't afford second takes of anything, so the preproduction phase consisted of meticulously crafting the shooting schedule practically down to the minute, writing every 'um' and 'uh' in the script, making sure the actors had copies of the final shooting script to memorize, rehearsals, etc.--basically, it replicated the live stage experience. And it was brilliant. Pre-planning made it so.
Two, by doing repeated takes and trying to work toward the best possible take for both actors, they tend to get a better working relationship with each other (assuming one of the actors isn't one of the 'top 10 actors who are hard to work with'), they tend to get a better understanding of their characters, the viewers tend to get funny outtakes in the DVD extras, the editors get more material to work with to make a better film, etc. More takes tends to make the film richer. Multiple takes make it so.
Three, by developing that technique, Lucas said 'Hey, now I can create an entire performance in the computer! Why bother having the actors on set at all when I can just paste their heads on this CGI body?' and we got Episode 3. That was terrible.
In all seriousness, though it was an interesting technique. It's reminiscent of voice acting work--VAs aren't in the same room, delivering their character's lines to each other. They deliver them to the sound booth, and then the director tells them how wrong they did it, and makes them do it again, but this time angrier. Then the best takes are edited together, ba-da-bing. It's part of the expectation among voice actors, but not for live film actors because it's an entirely different set of skills (not unlike the difference between film and stage, or stage and opera).