When I was young, a movie was based on a novel, a lecture was based on research and a joke was based on current events. But now movies are based off novels, lectures are based off research and jokes are based off current events.
Call me a curmudgeon
(“You’re a curmudgeon!”) but doesn’t the metaphor of a base suggest a foundation on which something else rests? Now I suppose one could argue that a base can also be a launching pad from which something lifts off. But to me that implies that the connection which once existed is now lost, in the past, left far behind.
Now if “based off” is questionable at best, “based off of” is completely out of bounds. The extra “of” is utterly unnecessary–a perfect example of clutter that desperately needs to be expunged.
Anne Curzan offers interesting analysis of this trend based on searches with the Ngram Viewer. She is a little more forgiving (and probably a better human being) than I am when it comes to recognizing that metaphors in English are not always strictly observed and probably don’t have to be.
Nonetheless, if I ever do something like that, it had better happen by accident rather than on accident.