Just wanted to add that yes, gold is a commodity, but it shouldn't be lumped in with oil, pork bellies or even silver (which is now generally considered to be more of an industrial metal, as opposed to a precious metal).
Yes, it would be nice to have data that goes back further than 1972, 1975, or whatever someone wants to consider as a fair start date, but at this point we have to work with the data that we have. In addition to financial systems suffering from inflation, they also tend to collapse with rather alarming frequency. One might prefer the term "macro shock" rather than "collapse" because something always emerges to take the place of the old system. For example, I know we are not Iceland, but I think their financial crisis of 2008-2011 is instructive. A portfolio that held gold still got whacked relative to the USD but it at least held it's overall value in terms of the Icelandic Kronur. That's just one example. A Brazilian or Argentinian investor would be well served to be holding some gold right now.
Gold is definitely a defensive investment in that one wouldn't expect it to do great when the economy is humming along. But economies go through cycles and diversifiers really help at certain times in those cycles. My take is that gold is a worthwhile diversifier to hold. Others won't agree and that's obviously fine.
One last point is that an investor's age does matter when talking about this stuff. I'm 57 and not particularly mustachian so riding out a 20-year stock market lull would be tough for me. Having a portfolio that doesn't potentially have a bad sequence of returns risk is important to me. If I were retired at 30 and able to live very frugally like a lot of folks on here, I might feel differently.
Sorry this post got so long! I'll crawl back into my cave now.