Isn't the phrase "for all intensive purposes" incorrect? Shouldn't it be "for all intents and purposes"?

Yup. Your intuition is correct. “intensive purposes” is indeed listed here on the “common errors in English” website

http://wsu.edu/~brians/errors/

Yes, and it’s very funny.

If you see someone make that mistake, tell him he should of known better.

Just kidding.

Hrrm- “should have known better, if one feels like picking some more nits:-)

Gotcha.

I should of remembered what a good writer you are. Gotme.

My favorite was a conversation between two coworkers. One was showing the other some pithy kitten graphic or similar kitsch. She said, “I think I’m going to put this one up. You know, just for chagrins.”

@ kevbo.. that’s great…
I was just discussing “all intents and purposes” just the other day

yes