"For all intensive purposes" is wrong. It was first written by someone who heard the phrase "for all intents and purposes" incorrectly. That is, it's something people who have misheard "for all intents and purposes" but who have never seen it written, have started using. It's meaning - to the extent it has any meaning - would be nearly opposite of the meaning of "for all intents and purposes." For all intents and purposes means, basically, "amounts to." For all intents and purposes, they're married, means that they may not have a marriage certificate, but they behave and live as a married couple. "Intensive purposes," to the extent it makes any sense at all, would be limited to a subset of purposes, i.e. those that are most intensive (whatever that might mean) rather than for "all" purposes and all applications, i.e. intents.