I fundamentally don't trust the people running the country (or states or locals, for that matter), don't trust the Fed, etc. Maybe "don't trust" is not the best word, a better description is that I believe they are all incompetent, or the system is so entrenched that its impossible to make any real change. (This is not political, it seems to be the case no matter which party is in power.) [Mod Edit]
I fundamentally trust the people running the country to keep doing the things that keep the money and power concentrated in the "investment" class. The people who run this country have a lot of stocks and will bail their friends out as needed. Those of us without billions have to be able to weather the times before the rescue arrives.
Otherwise, an ownership % of a growing cash flow is always better than a fixed cash flow. Since most companies have growing cash flow equity investors have done better than bond investors overtime, which is borne out by the numbers. There can be too much of a good thing, but that is why the suggested allocation is not 100%.