The reason this will never be implemented in tables is that tables, by their very nature, have text in parallel columns. There is no way to guarantee that the text in one column should break at the same point as the text in
other columns. Say you have several two-line paragraphs in one cell and several three-line paragraphs in an adjacent cell. How would you decide where to break the text? This becomes even more complex when cells are merged vertically. The only control you have
in tables is to prevent entire rows from breaking and, if necessary, dividing text into multiple rows to control page breaks.
This is the most comprehensive response that I have read so far.
I still wonder why w/o protection could not be calibrated in such a way as to (a) keep only two, not three, lines together, which (b) hit the bottom margin of a page, whether inside a table cell or not?