Personally, I don't like this strategy because, at least to me, it is contrary to the essence of sports and the spirit of the game where you are expected to always give your best.
However, fantasy sports are just that, fantasy and not exactly the same kind of competition that takes place on the field. Part of the essence of sport is the goal to win. How far you go to win is up to each team to decide, but as long as it is within the rules (which are up to each league to decide) some people will do whatever it takes to help their team win the ultimate goal.
While not exactly the same, you could debate the ethics of a team that has Tom Brady and David Garrard at QB. Brady's bye is week 10 and Garrard got hurt and did not play week 10. I would not fault the Brady owner for taking a zero at QB in week 10 (and losing that game), if he was confident that his playoff spot was secure and didn't want to drop anyone to fill the one week hole. Did that move affect someone else's playoff hopes? Possibly so. Do I see that as a "bush league" move? NO! One strategy is to intentionally build a team with bye week problems late in the season and bunched into one week just for the luxury of having the ability to make that decision.
In the end, pulling the stunt you described may backfire anyway just because it is impossible to predict the actual player performances that will take place in future games. I'll bet a lot of Favre owners were banking on Brett having a big game last night and look where they are today.