I think there's a constant tension when thinking about adding additional sources of income, because it's almost always more remunerative to pick the "best" source of income and focus all your efforts there. Even places where additional effort doesn't result directly in extra income (a salaried job, for example), it's probably the case that extra effort will produce more job security and larger raises--and that the total income will be greater than spending the same amount of time and effort on sources of "extra" income.

Having said that, I think diversifying income sources is a great idea, and I hope more people follow your advice. Focusing entirely on the "best" income source is a high-risk strategy that often pays off during good times, but in bad times, a diverse set of income sources can save a household all manner of difficulty.

It's best if people think through what they're doing, though. Otherwise, a simple-minded looking at the numbers can leave them disappointed with the results of their diversification efforts.