In the context of a personal investment portfolio, I'd question there are many circumstances where borrowing money to buy stock or whatever is a good strategy. This is not about building a business empire.
(One can reasonably debate paying down a low interest mortgage early vs. continuing to save in other ways.)
Note Harry Browne ran as the libertarian candidate for US president more than once and is really famous for
https://www.amazon.com/Permanent-Portfolio-Long-Term-Investm...
In my mind it is quite similar to Diallo's "All Weather" strategy where inflation protected bonds play a role similar to gold in Browne's portfolio.
On the other hand, the person who took out a big HELOC to buy Yahoo stock in 1999 shortly before being laid off was making a bet that ended up being life-changing in a way they didn't intend it to be.
(That's an extreme example but playing the averages both assumes that the average doesn't change and that they get enough rolls for the average to be a meaningful concept.)
If you just want to provide for yourself and your loved ones, leverage as a method of investing is quite risky and counter to your goals.
Of course, the “ZIRP” zero interest rate environment that predominated the 10 years since this article was written has been a historical anomaly. Though perhaps part of a longer trend, investors must be cautious not to view the benefits of recent leverage as evidence of easy future gains.
But if there was one thing that differentiates our financial position from our less-financially-free friends, it would be our comfort with debt as part of a well-developed investment strategy.
Debt is great for boosting returns, and the risk can be managed. But it remains risky. Certain strategies are less risky if you use debt. Those strategies have a risk that is astronomically above the risk tolerance these guidelines assume.