I think it's rhetorically effective, in that shallow thinkers have been trained for years to reflexively revile Nixon. If we argued merely on the merits, they might worry about LEO jobs or housewives hooked on patent medicines (i.e., stupid arguments we've seen before). It's best to short-circuit all that detailed disputation with a simple "Drug War = Nixon = Bad". Very few thinking people without a personal economic interest support the drug war [EDIT, quite right Josh:] with respect to marijuana at this late hour, so we really do have to worry most about the shallow thinkers for whom this sort of argument is compelling.