When I started beekeeping many years ago, I used to use inner covers but
after a while and after I had more then 4 hives, I found the inner covers to
have no real purpose, in fact I found that they help to keep too much
moisture in the hive which aids in the development of chack brood.
I have capped my hive inventory at 30 because of space and time but after I
stopped using inner covers I noticed that chack brood cases dropped to
almost nothing.  I keep bees in northern Utah and even in the winter I crack
open the tops about 1/8 of an inch to allow constant ventilation and rarely
see chack brood and do not loose any hives to cold.
Even for feeding I install an empty super on top and feed right on top of
the frames.
Another advantage that I found when I stopped using the inner covers is that
in the winter I can just left up the cover and can easily see the bees
without disturbing them through the empty super, they do not come up to the
empty super because it's a little cold up there and they just stay on the
top box within the frames.
The only thing with this situation is that if I take too long to install
frames into the empty super in the spring they start to build wax above the
frames, but this does not happen until April or May (depending on the
weather).
So, why use inner covers !.