These paragraphs seems to contradict. ConTeXt is useful if you use an environment more than once, but there are no ready-to-use ConTeXt environments.
You're probably confused by the term "environment". It means something very specific in ConTeXt, see for example section 2.3 of http://www.pragma-ade.com/general/manuals/cont-eni.pdf If by "environment" you mean some less down-to-earth, general document layout, e.g. for articles, books, presentation slides or letters, then yes, there are such ready-to-use layouts. Not always in the ConTeXt core, but they're there; for example, for letters you would use Wolfgang Schuster's letter module; for presentation, one of the solutions is Thomas Schmitz' and Aditya Mahajan's simple-presentation module, etc.
Yes, I meant RDF. XML is a very important format. I find it odd that TeX can generate PDF but cannot output simple XML.
Generating a document with a logical structure is really the opposite of what TeX does, at least in my view: in today's trend, you write a TeX document with a logical structure, and you generate PDF, a highly specialized format for describing the layout of a printed or on-screen page; it has rather few means of specifying logical structure (it's coming, but rather poorly supported by PDF producer applications at the moment). The same is of course even more true of DVI. Hence, what TeX does is to take a logically structured document, and to make it into a visually structured one. Note that I am not always convinced by the whole "separation of content and layout" creed which is heard very often as a selling point for LaTeX vs. MS Word etc., but there is some truth to it, and, generally speaking, you're still going from semantic markup to visual appearance, not the other way round.
So in order to have a semantical document I must write it in XML and then process it with ConTeXt?
Pretty much. ConTeXt is rather good at it. You have a lot of tools to process XML in the base code. There is also a module to deal with DocBook, but I think its development has stalled.
Is the capacity there (through LuaTeX perhaps) to write an XML generator?
The goal is that at some point, you will be able to redefine LuaTeX's backend, the same way you can act on the TeX engine at the moment. This will allow to control the output format entirely.
While I would expect the reasons for wanting XML output would be obvious
If I may, your statement might be biased by your own expectations. There are thousand people out there who use some form of TeX without even knowing about XML. But I know why you want it. As far as LaTeX is concerned, there are quite a number of toolchains that produce XML from some sort of restricted LaTeX markup. I'm not aware of anything similar for ConTeXt. Arthur