Extremism can be useful when used appropriately, but is not normally effective in its usual form which attempts to pull opinions from the middle toward one of the two edges or extremes. More effective form of extremism is Inverse Extremism which attempts to push opinions away from the opposite extreme. Inverse Extremism is effective because it relies on [negative] emotions to repulse subjects instead of logic or inference to attract subjects.
For example, instead of lobbying against abortion, one could form an Inverse Lobbying organization that propagates extreme abortionist views. In religious terms, this is equivalent to becoming the demon instead of demonizing others.
disclaimer: Inverse Extremism, as I have described it here, is entirely of my imagination. If this concept has already been describe somewhere else before, please let me know.