As far as I am concerned, your argument is just flat-out
wrong. People make the same argument all the time in baseball: "Oh, I don't watch the games in April and May because they just don't matter." That's exactly the attitude that can get a team in trouble. Sure, a team might not officially punch its ticket to the playoffs in the first four weeks of the season, but the way a team performs early the season -- and the events around the league, such as the way rivals perform -- sets up how important the games later in the season will be. Sure, there is a possibility that the 0-4 Vikings could make the playoffs, but the probability is minuscule. The fact that they have started out so poorly means that in the last quarter of the season, the Packers won't have to worry so much about season records, tiebreakers, and other permutations. The Lions' 4-0 start, by contrast, means that, barring a spectacular collapse on the part of either team, the race is going to be taut all season. It's likely that both season record and tiebreakers are going to be pivotal. Especially considering the fact that the Packers do not play the Lions until Thanksgiving, all divisional games at this point of the season are important. The Packers won't have a chance to notch the head-to-head advantage, so they will have to gain whatever advantage they can through beating divisional opponents and hopefully having the other teams in the division beat the crap out of each other.
So, yes, I will root for the opponents of other teams in the division, as well as for the underdogs in the division, because every time a divisional team gets defeated, the Packers' chances of making the playoffs improve.
What's the old saying? "My favorite two teams are the Packers and whoever is playing the Vikings this week"? Well, this season, it's the Packers and whoever the Lions are playing this week.