Honest: slabs don't lie and pretend to be anything else than what the other parts of the letter are (stems) = uniformity of the tracing.
Slabs are clean-cut in squares, which might hurt the eyes, some might say, but are an echo to our square world.
The whole question is: when a (letter) tracing is started, how does one finish the line ? Shall it imitate the pencil, the plume, the highlighter, the brush,... ? Why would serifs have a different shape than the stems just because they end the shape ? Slabs say no.
Geometric extension: slabs prolong, extend, continue the stems as if to encircle the letter in a box. One of the supposed reasons for serifs is that they give a sense of alignement of the upper line (x-case height) and base line and helps the reading. Slabs, in that regards, are the best reading support as their alignment service is the darkest (the blackest, since they consume more ink).
Slabs are so primitive, they look like what a baby would do to finish the shape, but also invite the imagination to finish it, as if the tracing had been interrupted.
Maybe they point to better shapes, more complicated, as perhaps letters should be.
Maybe they indicate that in our modern worlds, with our fancy eyes accustomed to zillion shapes a second, the mind is ready to accept (what would seem as) decorations within the shape, but ultimately a perfecting of the shape.
The beauty of Rockwell is in the geometric aspect, how could it not be? Could an egyptian not be geometric ?
Interlude: I just saw a Gill Sans sign - with its cut serif - the exact opposite of slabs: cut serifs say no, we need to simplify the shape.
Where would letters go if squared away, if looking like cubes (again how a baby sees letters) ?
