"Diversification is protection against ignorance," Buffett said. "It makes little sense if you know what you are doing." But he also said that 98% of people invest only in the SP500 which did better over a long term than other categories.
Diversification doesn't mean you have to own all categories such as growth, value, small cap, and mid cap, in the US and abroad. But that hasn't stopped many investors from owning 8-10-15 funds, keeping trading, and making less money. I decided more than 25 years ago to follow the money and let markets tell me where to invest. I used to own up to 5 funds, and now just 2-3 funds.
Started in 1995 to 2000: mostly SP500 + some SP500 growth. In 2000, I created the above.
2000-10: The SP500 lost money. I invested in Value, SC, and international
Since 2010: Stocks=US LC tilting growth to 2017. Since 2017 (retired in 2018) mostly in specialized bond OEFs + avoided all the meltdown (max loss from any last top under 1%).
The above shows that US LC dominated in 1995-2000 + 2010-24 while Value, SC, and international did well for 10 years. It's a proof that these are longer term cycles.
Here is an easy rule: if the SP500 does well, you must load. If the SP500 lags, you must diversify, starting with Value, SC, and international.
I know, you are going to ask, how do I know when to switch? You don't. How about looking at what the major categories have done in the last 4-8 months, twice per year, and deciding what to do? It's not science. You just do your best guesstimation, after several years, you get better. But, you will find pretty quickly it was a mistake to invest in international for 15 years. Remember, US LC value is a great option when the SP500 doesn't do well.
You also don't have to use all or nothing, you can play with only 30% of your portfolio.
I have never invested months in crazy, unique categories (gold, commodities, MLP, etc.), just wide-range funds. It amazing to me that someone who hardly trades starts trading in the craziest markets. You can't learn trading from an article; you actually have to trade, and only do it when you are good with years of proof.