...let me point out that both GUI-based word-processors and text-based
   formatters both have a language; one happens to be mouse- and
   action-based, and the other symbol-based.

True, but that's beside the point. This is a fact about an abstract model
of what the GUI users are doing, not about what they actually *are* doing.
This abstract model is only apparent from the perpective of a *programmer*
of the system. (NB: some users may see it, too, but only when they put
aside the work at hand and start thinking like a programmer.)

I'm not saying that the programmer's perspective is evil or stunted. After
all, that's what I do, too! I am saying that UI designers must carefully
distinguish between the user/programmer conceptual models, and they must
ultimately serve the user of the system, not the builder of the system. 


   I don't recall the actual stats, but something like 1 in 5 people can be
   categorized as a "symbol manipulator".

It would be interesting to know more about the meaning and basis for this
claim. At any rate, I don't think this is evidence that 20% of users think
like programmers. Bankers, financial analysts, structural engineers ---
these are all people whose work you could characterize as primarily symbol
manipulation. But what they do is not programming, and programming is not
required to do what they do. 