This is a soft question perhaps not well suited for the format of the site but I'm interested to hear opinions from this community on this topic.
K-12 mathematics textbooks (understandably) divide their content into chapters. My concern is that the level of compartmentalization from chapter to chapter is so extreme that it hinders the development of real mathematical thinking, and instead fosters the development of what I call flow-chart mathematicians.
The most discouraging example I know comes from the texts used in grades 7 through 9 in my own Atlantic Canadian province. Each book contains a chapter dedicated to "Real Numbers" where students learn to do arithmetic with fractions. In each book this chapter is followed by another called "Measurement", where children work on problems to do with perimeter, (surface) area, and volume. The thing that's shocking to me is that no problems in the Measurement chapter involve objects with fractionally labelled lengths!
I believe this is an error, and I sometimes try to address it when working with kids this age. After establishing the prerequisite skills, I'll draw a rectangle that's $4$m by $\frac{2}{3}$m and ask them to calculate the area. A surprising number of students who are capable of moving from $4 * \frac{2}{3}$ to $\frac{8}{3}$, and who are also able to explain to me how and why the area of a rectangle is equal to its length times its width, still have substantial difficulty putting these skills together in order to find the area of this given rectangle.
I think that this reveals that they don't understand that the multiplication of fractions that they've learned in the previous chapter is the same operation as the familiar multiplication of integers. Rather, they've learned a flow-chart to follow when they come across $\frac{a}{b}*\frac{c}{d}$. This, I think, is a huge problem!
I would like to hear arguments for and against this level of compartmentalization, and if possible, examples of situations where it's particularly beneficial or harmful.