While I agree with an indexing approach and follow one myself, classifying this in the same vein as income is a bit misleading, as another reply pointed out.
While income from side jobs can be irregular, returns from investments mainly concentrated in equities can be more volatile.
Volatility in the S&P 500 (as measured by the VIX) has been relatively quite low for the past few years, other than a few marked events. (US debt ceiling crisis, etc.) A chart of the S&P 500 shows more-or-less an upward trend over the past five years; certainly since the beginning of 2013 there have hardly been any wild swings.
This can lull investors into a false sense of security about the distributions of returns; just because returns have been steady (i.e. low volatility) over the past few years does not mean things will remain this way.
I don't dispute the long-term average yearly return of ~8%. What I am saying is that, as with many other things, averages don't tell the true story. One year, you could be down 30-40% in equities and another you could be up 30%. Getting a constant 8% return every year is unlikely.
For some long-term investors, they aren't concerned about the volatility and this is a perfectly rational thing. However, there are some people who cannot bear the volatility of such investments and will be in constant worry. Such individuals would likely have to apportion a higher percentage of their to less risky assets and perhaps miss out on some overall return.
I heard of many people freaking out just because the mini-correction that happened in mid-October. These sorts of people aren't cut out to be investing like this as they will panic and sell during downswings.