By Paul Yeager, author of Literally, the Best Language Book Ever
This mistaken phrase for all intensive purposes falls into the category I like to call “Close but no cigarette.”
The correct phrase is for all intents and purposes, and it means, more or less, “for practical purposes.” A correct example would be: For all intents and purposes, the game was over in the fourth inning when the Yankees scored 10 runs.
The commonly used incorrect version of the phrase (for all intensive purposes) seems to indicate a sense of urgency, such as an intense situation. This is contrary to the original use of the phrase.
A Google search for the mistaken phrase results in 17 million-plus results. Granted, many of those are entries that point out that it’s incorrect; however, it is a general indication of the widespread use of the incorrect phrase.
For more information, see this Wise Geek entry: What does “All Intensive Purposes Mean?”