> So my question is whether the main benefit of upper entrances
 > is due > to their giving the bees more ready access to an
 > entrance for defecation flights other then having to crawl down
 > through cold abandoned combs.

That is what I have been saying.  I also see colonies with upper
entrances stay more even since the weaker colonies can maintain
entrance activity and do not lose bee to stronger colonies on
flight days. (Lost and orienting bees are attracted to colonies
with entrance activity).

My wintering hives are often three and four boxes high and that makes
for a long hike to see what the weather is like and I think that in an
insulated hive with no top flight hole, the bees can miss the short
early tree flows.

The upper entrance also allows old and sick bees to make a clean exit.

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