The rack basically provides extra room for the bees, in which they cannot
construct comb.  That is, the spacing between the bars is a bee-space, so
they have some room to cluster without letting them "build down" off the
bottoms of the brood frames.  In section comb honey this is important
(using the traditional Killion/crowding plan) so that you can have lots of
bees crammed into a brood chamber, with no choice but to go up and work in
the sections.
 
On the other hand, the commercial "flat" bottom boards are only 3/8" or so
deep, and so provide a fairly narrow entrance.  Yet the bees do well with
them, too.
 
I haven't used the s. racks, but at one time built "tunnel entrances" for
my comb honey colonies.  The tunnels were in effect the front, solid
portion of a Killion slatted rack, and jutted into the entrances about 4"
or so.  Fitted into the 7/8" (deep) side of a standard bottom board, they
provided a 3/8" high entrance.
 
The tunnels helped to reduce the gnawing away of the bottoms of the brood
combs, so that the single brood box could contain a full complement of
quality cells for the queens' laying.  Made out of masonite, the tunnels
were chewed away pretty well after a couple of seasons, and I didn't bother
with them anymore.
 
The racks are probably quite beneficial on any hive, but still, it's
another piece of gear to worry about...
 
Best regards,
 
JWG
Freeville, NY