stocks
bonds
real estate
cash
gold
miscellaneous
cash / currencies

We all handle cash every day, most of us receive our salary in cash (instead of maybe stock options or food and lodging...). - cash as in money you have in your pocket, and in your checking’s account is just daily business.
And thus not worth considering in such profound decisions as in how to handle our investment portfolios…

But handled in a way to keep abreast with inflation, cash can be a most effective diversifier, as it has by definition a very low correlation with almost every other asset class.

And for those who are holding any asset classes in another currency then the one they earn their money in, the currency markets become an issue as well.
Holding cash in different currencies can really mess up simplicity in your portfolio, as it adds another layer of low-correlated asset classes, in an even less predictable market. But low correlations always make for good diversification, so it should not be dismissed off hand.