They posit a set of definitions. They explain how their definitions elucidate things. That's fine. They present an argument for a set of definitions. They really have to stop there or be very, very careful. Seldom are they sufficiently careful. They invariably assume, on some level, that their definitions are universal and the Only True Definitions.
In so doing they exclude lots of things from the discussion for the sake of simplicity, then tacitly reintroduce them for the sake of self-righteousness.
Once saw a sabbath-keeping tract that was silly: The Sabbath is the 7th day, Saturday; the Puritans said they kept the sabbath; therefore the Puritans went to church on Saturday. They posit their definition. They assume that their definition is universal. They apply their definition and forget the Sesame Street Song, "One of these things is not like the other."
At the end the reader is either nodding because it's always as they've always believed and they like nothing more than being told how good and smart they are; or they know something's wrong, words have been twisted and they're still sure they're right they just don't understand how they've been made to sound wrong--and they're not savvy enough to parse the illogic. It's hard to understand that different groups have different, competing definitions for the same word and that those definitions can shift over time.
Nicholas Kristoff's NYT column today plays the same silly game to reach the solution that he fervently believes has to be right--otherwise he's wrong, and that's simply impossible. (
http://www.nytimes.com/2011/03/06/opinion/06kristof.html?ref=opinion ) He assumes that there is and always has been just one "Islam." Some sort of Platonic form, unchanging and unalterable--and that anything else can't really be Islam. He's being ridiculous, of course, but it's a really common trait and one that very few people seem to notice.