what's considered correct usage in English is very much driven by common usage. the language evolves. many people (currently erroneously) write "should of", this is likely to become the correct way of shortening "should have". just like an apostrophe will eventually correctly denote a plural (it already does this unofficially, even in formal documentation) - I'm sure there are other examples you can reference yourself (such as the increasing substitution of the verb "to say" by the compound verb "to be like", as in "I was like", "he's like", etc)