There are many things that fit this description: being a parent, puberty, your first kiss, falling in love, a death in the family. Would you cling to a definition of yourself that has no room for major, perspective-changing, life-altering insights? You can try, but life has a way of changing constantly whether you want it to or not.
Recreational drugs serve no "purpose", as you say (objectively useless). But what purpose is there in being born, eating, making love, making friends, and eventually dying? Purpose is what you decide it is. If some people want to explore their brains using chemicals then what the problem with that? Yes, everything has risks, but so does getting up in the morning.
Live and let live. You may not like the fact that drugs can change personalities in a way that makes you uncomfortable, but this is a big world: I'm sure you will be able to find like minded company. Maybe even here on HN? Who knows.