"Language evolves" is a much overused explanation/excuse. Not all changes are good. Like that bloody "big of a deal" that's taken hold in America - explain to me how that improves the language! I hear that "of" sneaking into other sentences too; HATE IT HATE IT HATE! And "could care less" as well. You could use "language evolves" to justify that as well, effectively making "could" mean "couldn't", like people use "literally" to mean "not literally". These are bad changes, in my opinion, and should be discouraged.
To be fair evolution is not always good (though using 'good' in this context is in itself an example of language changing since the context is not about moral goodness) from an outside perspective. It is molded by the needs of a group.
Edited by Mirrimon