The importance of blasphemy is in providing a language of power. To decree certain views, certain ideas, certain practices, even certain thoughts, as taboo is to demand that certain forms of power cannot be contested. The importance of the principle of free speech is, on the other hand, in providing a permanent challenge to the idea that some questions are beyond contention, and hence in acting as an ever-present test to authority.
This is why mockery is necessary. It is not enough to respectfully disagree, because the people in power will make simple disagreement disrespectful (as they have done before).
We must be free to mock, to jeer, to point and laugh at figures of power and authority; or we will not long be free. The American system recognizes this; it is why every President attends an annual event where the press makes fun of him, to name just one of the many traditional and institutional defenses of mockery.
Conspicuously missing from most religious traditions, however, is the duty or the right or even the ability to question basic premises.