Assuming Bad Faith edit
Two of the espoused main tenets of Wikipedia, Assume Good Faith and do not bite, have their basis cemented in willful naïveté, unicorns, and gum drops. I have yet to encounter, or read about, a society which endorses purposed destruction of their constructs; it's simply considered illegal and all except the most cave dwelling of their inhabitants understand this from a very young age. Yet, at Wikipedia we are to believe that these same society occupants have honestly forgotten their moral system and aren't aware that vandalism is disallowed.
What Assuming Good Faith and a system of graduated warning levels (as opposed to immediate blocking) introduce is the fairy tale concept that a person who thinks it's a valid idea to write something like 'suck my dick' in an article will simply see the errors of their way and begin contributing helpful tomes to the Wikipedia effort were they only given a polite reminder that their edit is unacceptable.
While the percentage of people who are theoretically asshat-turn-contributor isn't exactly 0%, it is likely damn close; one has to ask whether the cost of time and sanity of vandalism patrolling editors who need jump through numerous administrative hoops and rounds of warning is worth that near 0%. To me, the answer is clear.