Originally used in common American/English vernacular as 'all intents and purposes' by just barely unintelligent people trying to sound as if the 'intents and purposes' surrounding them are highly important.
This phrase has since been changed to 'all intensive purposes' by people who do not listen carefully to the original phrase used in poorly written movies and who have also never seen it in print in equally poorly written books.
Also, the person listening to a speaker who says 'all intensive purposes' is likely to overlook it.
Tanar: Yeah, you're pretty much by best friend, but for all intensive purposes we'll just call it a tie between you and Kelsey.
de facto: The phrase is a corruption of "for all intents and purposes" by persons who have heard the phrase, but have not read it in it's proper form. It means "for all intents, and for all purposes."
de jure: Taken literally, the phrase means "for purposes which are intense. All purposes which are not intense are not included." This is almost completely opposite to what is meant by most people, and is why it is imperitive that persons use the proper phrase.