You're right in the fact that it isn't really going to do all that much but it's going to do something.
A portfolio's overall objective should be to be greater than the sum of its parts. What I mean by this is holding a wide variety of diversified assets should ultimately reduce your risk and the holdings overall should work towards a grand "goal" for each investor.
A single individual equity is very risky, for example. However, a portfolio of 25-30 equities has significantly lower risk despite each equity still being risky (assuming you've identified strong companies, of course. You could certainly hold a portfolio of 30 terrible stocks if you really wanted to).
So, for this reason, gold would simply be another cog in the wheel of your overall portfolio. No, a 5% position won't prevent significant volatility during a global crisis. But it will provide some reductions if theories hold true.