Appeals to dictionary definitions are annoying because they imply that the language we speak should be prescribed by, not described in, dictionaries. That language is not living and mutable. Many of my students regularly appeal to dictionaries, goaded by a love of 'fact', even for terms which we have discussed extensively in class. Equally, appeals to dictionaries have been numerous in responses to elmyra's post, "I am an Eastern European".
Annoying as they are, I am, at the minute, more curious about the history of this rhetorical device than I am in eradicating it. Dictionaries are useful sources of consensus about word meaning at the time and place that dictionary was printed - which is why the argument is vaguely more compelling if one provides the name and date of publication of one's chosen dictionary - but they are not strict determinants of meaning; at least, not outside of games with specific rules to the contrary.