You should absolutely understand the very large chance that your equity is worthless. You should absolutely 100% not plan any part of your life around the equity being worth something. You should understand that, even if the equity is ever worth something, it won't be liquid for a very, very long time.
...but all of that is different than it being worth nothing. I feel like the "your equity isn't worth anything" mindset leads to employees allowing startups to give them smaller amounts of equity than they should. It leads to employees not questioning bad practices (like 3x preferred participating shares) as much as they should.
I've had people quote the "equity is worth $0" thing to me in negotiations about equity/reups. In my head: "Oh that's actually great! You can just give me your equity then instead of me having to negotiate a reup with the company."
Said another way: let's say you find out your startup's VCs have participating preferred shares. There's a world where that's fine -- you just now need more equity to get to the same place as you would in a company that had non-participating preferred. But you need to reason about the value of your equity to reach that conclusion. "Equity is worth $0" discourages thinking about that stuff.