I run my 8 colonies with top and bottom entrances. Bottom entrance is normal full width of box, top entrance is a 2" slot cut in the inner cover on the underside which is intended for ventilation. A few bees stand guard up there all the time, there is a bit of traffic in and out for supplies, but mostly they seem to prefer the wide space of the bottom entrance. As winter prep they tend to accumulate plastic propolis near the top entrance, which is used to block off the gap in the inner cover. The top entrance is never fully blocked by propolis. I take from this that the bees see an advantage in the ventilation provided by the top entrance.
What is a puzzle is the degree to which individual colonies block the top entrance. Some have blocked it off leaving enough passage for two bees, one on each side of the slot. Others have blocked it off hardly at all. My colonies are pretty even in terms of stores and population of bees, so I'm wondering what other factors might be involved in their decision to what degree to block the top. I was thinking perhaps other ventilation gaps (between boxes, through telescoping cover, etc) but again I don't see a correlation between hive integrity and degree of blocking.