> Also, any pro barista will tell you that espresso is NOT actually better with a "single-source"...

As a former barista (incl competing) and commercial roaster, "coffee bon vivant", I agree with you (it's a rare pleasant surprise that a single origin really works out of a commercial espresso machine, IMO), but there are lots of reputable shops/baristas that will beg to differ. The whole universe/culture is big enough though that after my ~20+ years in so far I'm still open to being surprised.

I usually describe espresso coffee (the beans) thusly (crude, but effective(ish)):

  The opposite of scotch - the highly regarded scotches are single
  malts, and you don't want to distract from their quality. Blended
  scotches are concocted to fill voids or weaknesses in in the
  collection of ingredients that won't stand alone. In espresso,
  there is no single bean that will provide the body, acid, flavour
  that makes the best espressos, so you necessarily blend to collect 
  the profiles you want into a single product (the coffee blend
  you'll use). Some even use beans that (in N. America, at least)
  we're advertised-to are inferior (robusta[0]), but even these
  have desirable properties[1], in the right hands.

Re: "terroir" -- I think that's a wholly appropriate term to use. I've got friends that work at Stumptown (and other smaller roasters) and I get to sample various batches through the process of zeroing-in on a final roast profile, and try beans from the same estate, but various lots, year-over-year, etc., etc. and sometimes the changes are jarring. It's a very interesting universe, not unlike wine.

Edit: clarifying words, formatting

[0] https://en.wikipedia.org/wiki/Robusta_coffee

[1] http://www.coffeegeek.com/opinions/cafestage/02-01-2006