I do see that this is the TextEdit design (but not the design for Mail or Safari).
The problem is that it forces the user to shift gaze, search for the insertion point, and refocus on text. That is, when one presses the down arrow, one’s eye follows the insertion point one line at a time, because that is the location of interest. After one reaches the bottom of the window, pressing the down arrow causes the window text to jump ½-way up in the window. The user then has to search for the insertion point. That eye movement is disorienting and wastes time. In my experience, this design choice (for text/word processors, but also for many other applications) is very much in the minority.
I can’t think of a situation where moving a ½-window makes sense. When skimming (i.e., not reading in detail), one wants to move a full window at a time. When reading each line, the only jump that makes sense is at a page break. Anywhere else is disruptive to reading.
Does Apple provide any configurable options?