The other not-too-unusual case, where it really /is/ an .RTF that just happens to be /named/ .DOC, those it really oughta open, IMO. If it doesn't do that, then I'd take that fact as proof that this was /not/ about security, but deliberate crippling in order to enhance sales.
(And yeah, yeah, people may point to the free[-as-in-beer] Word Viewer all they want -- but when one program is replaced by two like this, I don't think that's for the /customers'/ convenience. Does that Viewer thingy even come with Vista? For free? With all eleventy-seven editions?)