According to http://www.alphadictionary.com/blog/?p=294
the use of "off of" is dying, not growing, as off becomes more, um, prepositional and less adverbial. Among other roles, the words "off" and "out" used to function as adverbs and still do for many of us Americans. As adverbs, they require a preposition.
So for your consideration:She ran quickly.
[Fine - quickly is an adverb.]She ran quickly the road.
[Wrong - quickly is an adverb! Where is the preposition?!?!?]She ran quickly down the road.
[That's better! The preposition made it all better.]She ran down.
[Incomplete - could be fine in context, but the prepositional phrase is missing an object.]She ran down the road.
[Fine - down is a preposition.]She ran down on the road.
[Wait, now you've got two prepositions and only one object. There had better be some context to supply the missing object!]She ran off.
[Fine - if off is an adverb.]She ran off the road.
[Fine - if off is a preposition, but if off feels adverbial to you, then you feel like "Ack! Where is the preposition?!?! I need a preposition!"]She ran off of the road.
[Fine - if off is an adverb, but if off feels prepositional to you you ask "Now why is that idiot using two prepositions where one would do just fine?"]
It's not just "of" that works. If off is being adverbial, it can play with most any preposition.She ran off on the road.She ran off along the road.She ran off with the road
[Ok, so this doesn't make sense, but only because its hard to move a road. Just picture a six-year-old girl with a piece of road from a toy set.]She ran off to the road.She ran off down the road.Conclusion
: The debate should not be whether or not to use the "extra" preposition of with off, but whether to use off as an adverb or an adjective.
Also, if you doubt that "off of" is ever different from "off", consider these sentences (if you'll excuse a common synecdoche) and which you would rather do.I just pissed off the White HouseI just pissed off of the White House