Microsoft can already get stuff built to its own specs,
MS specs are not hardware designs. They're a basic list of hardware requirements, but MS traditionally relies on the engineering expertise of others. ODMs.
as does Apple and everybody else
Not at all like Apple. Apple actually designs all their stuff and farms out only manufacturing and assembly.
What value does Nokia bring to the table? Are they really better at engineering hardware? Are their distribution channels notably better? Is the Nokia brand that big a valuable property?
All three of those a resounding yes. IMO, the other producers of Windows phones only serve to devalue the brand MS hopes to build. With Nokia, I'd also suggest forgetting the WIndows moniker and build on something brand new, since for all intents and purposes, the product is new - at least looks and feels new along with needing all new third-party software.
I think RIM does realize that they're trending downward. That's one of the reasons for the big tablet play. I'm actually surprised they didn't go full Android on that thing. I think they'll be able to work it out and hold on to third place ahead of the others.
With Nokia, MS should be able to hold 4th place ahead of HP/Palm.