Yes, communal feeding stimulates robbing, especially during a dearth.
 
Another important reason for not using communal feeders is that the
strongest colonies will get most of it, and the weaker ones (which
could benefit most from the feed) will get a much smaller proportion.
By putting the feed directly in the hive under a spare super, you
make sure everyone gets what they need, not necessarily what they
want (to paraphrase the Rolling Stones).
 
Colonies vary a lot withrespect to how fast they consume their stores.
It is important, and good management, to check them out before feeding.
Some might need several gallons, others virtually none.
 
Joel Piepenburg asked about the black mold.  I too see it, and I am curious.
So far I have seen no ill effects, but I do make sure that I clean
up the jars well between feeding, and use plenty of hot water.
 
Also, I use the largest entrance on the bottom board year round
in Maryland.  I have been taught to keep an upper entrance also
at all times.  This is a notch in the rim of the inner cover, about 3/8 by
2.5 ", and the inner cover is put on with the notch down - going
directly into the brood chamber.  The gap between the hive and
telescoping cover provides a weather-proof entrance.  Make sure the
outer cover is always pushed forward.
This provides ventilation when feeding, and keeps the hive dryer in
winter.