I've worked in teams bad enough to make code reviews a real pain.
I've seen pretty horrible code constructs that actually seemed to work without bugs. What should I say? That the author of that piece of code should completely rewrite it to my taste? It would not only frustrate him/her but also slow down the entire team. So I often accept what is rubbish IMAO. And not me alone, have you ever seen two code-buddies in a team always doing each others code review and(quickly) accepting and merging everything they produce before anyone can make a comment?
I've had numerous pointless discussions about why I wrote a specific piece of code where (a junior dev) the assessor just wanted me to do things by the book as he learned it, not really understanding why I did it that way. But there was no way to convince him, it ended up in an unresolved conflict that I only could solve by messing up my code to his taste.
All these pointless discussions that particularly affect the relationship you have with your co-developers in the team you work in, are not always fruitful and can be quite destructive if you're not careful enough.
I do believe code reviews can be very important and valuable, but at the same time I've seen huge amounts of wasted time and frustration for nothing. It's not always and only a good thing is my conclusion, it depends on the variables.