Given that the goal is reducing injuries, let's think about the repercussions of forcing each of your starting o-linemen to sit down for 4 out of 20 games. Do you rotate them, with one of the 5 sitting out every game, so your ostensible starting line NEVER plays together and you NEVER field the same OL 2 weeks in a row? Given the critical importance of consistency and communication on the line, that sounds like a recipe for getting your QB killed.
So perhaps you should choose the 4 least important games of the year and sit them all together? But you can't risk standing your franchise QB behind a scrub line, so you throw in a scrub QB as cannon fodder. At this point you're conceding the game, so might as well rest some other key players too. In essence, you'd end up playing the 4 preseason scrub games but having them count.
Another question: when do teams have to announce who is sitting for a given week? Whenever the announcement comes, it's problematic. If you announce your "outs" at the beginning of the practice week, then an injury to the "ins" leaves you high and dry. But if you wait to make the announcement, then you face all sorts of subterfuge trying to run practices that legitimately prepare the players who will be playing, without letting on who is really going to be active. Not only is the football impact enormous, but the gambling impact is too. You'll need some counterpart to the injury reporting system.
(And if it's a gametime decision, then you have the network running promos all week straight into the pregame show hyping "Brady vs. Manning!!!"...but then neither player suits up. The networks would go ballistic.)
