I think it is understandable, perhaps some managers are worried and prefer to avoid the risk of their players getting injured, so they do not take the Club World Cup too seriously. If that happens, it will certainly disrupt their journey towards next season. However, the Club World Cup can be a very good event to build chemistry between players, and also with managers. In addition, each manager can certainly give more playing opportunities to his second string players.
The injury can be caused by many factors, it still can happen even if they don't join FIFA Club World Cup. We also still have enough time for the break and preparation anything. So it is actually no problem to join FIFA Club World Cup. Yep, it is a chance of the second tier players to play a longer time, but there are many other advantages of this competition. The new players can start to adapt with the club games, the coach can evaluate the current tactic/strategy, and the club owners can see the capability of their current squad. And they also can have a chance to win a trophy of this competition.
Bayern Munchen have been very impressive so far and are leading Group C, while Dortmund look to be in second place in Group F, but they have the same number of points as Fluminense. Overall, I think both Bundesliga representatives will easily reach the round of 16, both will probably be more serious when they are in the knockout stages.
I am not surprised if Bayern Munich can lead the group C with a perfect result in every match because they still don't face a strong team yet. Tomorrow will be a more interesting match because Bayern Munich will play against Benfica. This match won't be as easy as the previous matches if Benfica plays with their main team. However, I think Bayern Munich will still lead the group C at the end of this group stage. Meanwhile for Dortmund, they must win the latest match in the current stage because they will only play against the weakest team in group F (Ulsan). Dortmund will go to the next round together with Bayern Munich. Sure, the round of 16 will have more serious matches because those strong teams will face each other.