> Can anybody account for the value of fumigating 
> or treating for Nosema (south of Canada)?

This question simplifies down to "what do you do with a deadout?"  
To each his own.

Most large operations cannot fumigate, as they would need the ability to
fumigate "on the road", or they would need to drag the deadout back to a
central location for processing.  Those of us who would be called
"sideliners" (less than 5K colonies) or who are regional, rather than
transcontinental beekeepers, are more likely to be able, as our distances
are smaller, and our scale is smaller.

At the high end of professionalism, one could autoclave each and every
deadout, and thereby (literally) sterilize deadouts, at the cost of losing
all the drawn comb.   At the low end, one can make a split, and install the
spilt directly into the deadout hive, without even inspecting the brood comb
in the deadout.  The downside risk to a lack of "comb biosecurity" is being
caught flat-footed by the next pestilence to wipe out beehives.  And if one
is "wrong", and another pestilence never comes, one at least has somewhat
"healthier comb", which certainly cannot hurt. 

I dunno anyone who is so straightedge-hardcore as to autoclave 100% of
deadouts, or so lackadaisical to not even look at the brood combs of a
deadout before shoving a split into it.  So everyone is rationalizing some
sort of "cost" of triaging/culling/handling combs from deadouts.  The
variable is how much time and money does one spend beyond simply removing
the least appetizing-looking combs to make room for a split.

There was a time when a great number of things were being intoned by
self-appointed authority figures that would lead a rational businessperson
to conclude that "better sanitation" certainly would not hurt in efforts to
keep one's count of living colonies high enough to avoid defaulting on
pollination contracts or having to beg/borrow/rent colonies to perform one's
contracts.  A lot of these pronouncements had to do with Nosema ceranae, but
Nosema was and still is a mere placeholder for "The Next New Pathogen".  One
is wise to establish sanitary protocols before the next scourge of bees and
beekeepers arrives in a shipping container from a far-off land.  Look at all
poultry operations do in a post-bird flu environment to avoid losses.  (And
if I am a "beekeeper", why isn't a poultry farmer a "chicken tender"?)

And it makes sense to take generalized action to treat brood comb as a
vector for more than AFB. When the core findings of a much-ballyhooed paper
published in "Science": 
"A Metagenomic Survey of Microbes in Honey Bee Colony Collapse Disorder"
http://doi.org/10.1126/science.1146498

were quietly contradicted and retracted via an article in beekeeper
magazine, ABJ:

"Historical presence of Israeli Acute Paralysis Virus in the United States" 
(Yanping Chen and Jay D. Evans, American Bee Journal 10/2007)
https://in.gov/dnr/entomolo/files/IAPV.pdf

I was happy that I had sold my entire operation well before any of the
nonsense started.
But even with only a "hobbyist" colony count, it still made sense to try to
control pathogens:

a)  Recycle all brood frames, 2 per box per year, using queen colors to
track frames, so that no brood comb serves more than 5 years

b)  Treat deadouts as potentially infectious items.  Remember, no one knew
for sure why "CCD" wiped out some yards, but not other yards

c)  Do what one can with the extra brood boxes removed in spring when
preparing colonies for pollination (or, in a hobby-beekeeper mode, reducing
the total comb area prior to putting on Ross Round supers, which have only
become that much more valuable as they have become a niche product).

Note that in (c) I was/am "crazy", and use only medium boxes "Friends don't
let friends lift deeps", as I say.  So, overwintering in 3 mediums
(equivalent comb to 2 deeps), meant that one box and 10 empty combs has to
come off in very early spring,  to be replaced with a hive-top feeder to
create a standard-height hive that could be prepared for (and survive!)
apple pollination in the rain, chill, and mud of spring.  One had to do
something with those extra brood boxes before using them to make spring
splits, so why not fumigate "just to be sure"?  They had to be stored, so
the marginal cost of fumigating was no more than the chemicals, and some
rolls of pallet wrap to make each stack airtight.

Typing all of this may illustrate how unique each operation's choices can
be, and how customized each operation can become for each beekeeper's own
idiosyncratic business plan.   
 
As an aside, one does not retract a paper published in the journal "Science"
via an article published in a magazine for beekeepers like ABJ, but this was
the same gang who stared intently at varroa mites for over a decade before
one fellow noticed that the mites were not round, but oblong, something that
could be observed with the naked eye.

"Varroa jacobsoni (Acari: Varroidae) is more than one species."
Anderson DL, Trueman 
http://doi.org/10.1023/A:1006456720416

In their defense, just about everyone involved in the "Metagenomic Survey"
paper rightly felt "stage managed" by Ian Lipkin into being a bit player in
what seemed to be a Cecil B. DeMille-produced extravaganza about Ian leading
to a (still unrequited quest for a) MacArthur Fellowship.  The moral of the
story is that new toys can "discover new pathogens" that have been floating
around harmlessly for decades, and that "post hoc ergo propter hoc" is still
painful, awkward, and fatal to credibility.

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