I don't agree. There's an art to story telling. You can ameliorate the material without dramatically changing the origina story line. And you should, unless you want to take the knock and actually call the movie an adaptation or acknowledge that it's only based on the source material. If you're using the name of the original book, any changes for the sake of the movie should be for the sake of advancing the experience or be required because the mediums are different.
In this case, the storyline was changed in order to spread the source content over an additional four or five hours in order to make three movies out of what should have been one or two. What's worse, it was sloppily-done, because the added scenes could easily have been added in the form of flashbacks in a parallel storyline that would not have required changing the original so dramatically. It was authorial laziness and not a necessary compromise that was necessary in order to bring the childrens' story to the big screen.
Saw 'This is 40' this weekend, too. Scary how much it looked like my own marriage. Even the little details. The wife was in stitches--people were looking at us. I thought it was a bit spotty and long, but a good flick for parents in their early 40s.