When I decided to go to graduate school (art history), I spoke to number of
people whose opinions I respected (curators, directors, professors, etc.)
Someone told me that earning a master's degree is not really that big a
deal, and that the PhD is what matters down the road.  They said that the
master's is required as a minimum by the top museums for just about any
post, as it shows that one can research, write, move in academic circles,
speak, handle stress, and meet deadlines...rather than showing any type of
useful art historical knowledge.  So, I opted for a traditional art history
program, and one month after I received my degree, I was offered a post in
my field at one of the top art museums in the US.

So, when you ask why an advanced degree is required, perhaps it is not only
because of the "knowledge" it gives, but also because of the skills required
to earn the degree.  I suppose, Deb, that that is one way in which "learning
something" can be equated with the ability to perform on the job.

On another note, I quickly realized the WHAT you study often has less impact
than WHERE you study, particularly at the big name museums.  I guess it kind
of fits in the logic of "why hire someone with a BA, when you can hire
someone with an MA?" , "why hire someone who went to Joe Schmoe College when
you can hire someone who went to Big Prestigious University?"  When hiring
an employee, the goal is to limit the field of applicants in order to make a
decision.  Whether someone with a BA knows as much (or more) than someone
with an MA becomes irrelevant.  The goal is to select one candidate from a
field of many, and looking at degrees, experience, universities, references,
etc. is how such determinations are made.  This is true in all fields, not
just museums.