Movement Capital had an interesting post (with data) that concludes small portfolio tweaks to things like gold or REITs, both as examples of diversifiers, don't impact returns very much. I agree with some of what they say and disagree with other parts of their thesis.
The post starts looking at smart beta funds like Vanguard Value ETF (VTV), Invesco S&P 500 Low Volatility ETF (SPLV) and iShares Edge MSCI U.S.A. Quality Factor ETF (QUAL) versus just owning a single cap weighted, total market equity fund and they're right, not much difference over a longer period of time. This is an area we've addressed before. Various smart betas strategies can of course outperform, they can also do what they say they're going to do (like low volatility smoothing out the ride) but to the extent some smart beta funds are simply a slight rearrangement of the same names in market cap weighted funds (overly simple description), they are unlikely to have radically different performance profiles. The research cited by Movement Capital supports that idea.
I would contrast the above with my experience using diversifiers in the financial crisis as well as in Q4 2018, both shorter term events than looking at a ten year run like Movement did. First the financial crisis with the diviersifiers I used as the market was rolling over;
They all generally did what they were supposed to, they did not look like the stock market at a time when their potential zig zag effect mattered most. It's not a stretch to see where small allocations to several of these could help offset the full brunt of that large decline. I prefer several small allocations to various alternatives in case something unexpectedly goes wrong with one of them, the one I might have gone heavy in.
Similar study from whatever it is we're calling the decline in Q4 of last year;
As we've discussed countless times, the domestic stock market has an up year the vast majority of the time and if you capture most of those gains while maintaining a reasonable savings rate you have a pretty good chance of having enough money when you need it. Avoiding the full brunt of large declines then is nice but not crucial. It is important for me to help clients avoid getting to a point of panic and to reduce the impact of any large withdrawal that they need to take.
Circling back, the market goes up most of the time so it is above its 200 day moving average most of the time which means there is not a need to have a heavy allocation to diversifiers most of the time. Defensive positioning is more of a shorter term strategy not ten years as indicated in the above linked blog post.