Re: should you smoke marijuana on your aikido journey
The argument being made takes the form, "Two wrongs don't make a right," but this was not the point being made.
Accordingly, and apart from the question of whether smoking pot, drinking whiskey, or huffing a cigar are morally like adultery -- I agree they are not -- another question arises whether there is a meaningful difference between justifying a behavior (e.g., since politician x does y, so can I), and asking folks whether they'd be equally as prepared to condemn a comparable behavior in which they or someone they know also may engage. (E.g., if you're willing to condemn people for x, are you also ready to condemn people for y?) It's an argument for lenity, not one for justification or even for excuse.
Most people, I bet, would agree that at least some of the time someone sensibly could say, "I don't find 'x' justifiable or even a good idea -- nor am I willing to condemn it. Live and let live."
Unless a person is willing to argue that everything which is not "justified" should be "condemned," these describe two different, if overlapping sets of behavior.
There are a number of words in English -- as I am certain there are in most languages-- for someone who doesn't make this kind of distinction, and those words normally are not a compliment. Think of the character of Inspector Javert, persecuting Jean Valjean for stealing a loaf of bread, for example.
Acts that are unjustified and acts that must be condemned simply are not mutually exclusive categories in everyday thought, because most people don't demand moral perfection.
A closer analogy would be, "Should politician x condemn politician y for committing a particular act when x has done the same or its equivalent?" It suggests, I think, a different answer to most people.