Why do people use *fallacious reasoning?

*for those of you who don't know what fallacios means... it basically means misguided

•containing or based on a fallacy; "fallacious reasoning"; "an unsound argument"
•deceitful: intended to deceive; "deceitful advertising"; "fallacious testimony"; "smooth, shining, and deceitful as thin ice" - S.T.Coleridge; "a fraudulent scheme to escape paying taxes"
•based on an incorrect or misleading notion or information; "fallacious hope"
wordnetweb.princeton.edu/perl/webwn

•A fallacy is an argument that is not logically valid. The truth of the conclusions of an argument does not determine whether the argument is a fallacy.
en.wikipedia.org/wiki/Fallacious

•Characterized by fallacy; false or mistaken; Deceptive or misleading
en.wiktionary.org/wiki/fallacious

•fallacy - a misconception resulting from incorrect reasoning
wordnetweb.princeton.edu/perl/webwn

•fallaciousness - result of a fallacy or error in reasoning
wordnetweb.princeton.edu/perl/webwn