Many people depend on one source of income whether that be Google Adsense, eBay, etc. What will you do if your only source goes away? Diversification is key in business guys.
I agree. It's kind of the same thing with stocks. You don't want to have all of your money invested in just one stock. It is good to have multiple sources of income. It is even better to have multiple sources of passive income.
I like to invest in both long term strategies like SEO (mainly affiliate websites) as well as short term income like paid advertising campaigns. I guess key is to never stop learning and being curious about new traffic sources and new internet marketing strategies in general.
You also need to consider diversity of lead generation (i.e., customer generation and/or traffic generation) for your business. Have a well-balanced "diet" of marketing techniques is also key.
You can't create business that relies on another business to pay you. I.e Google Adsense. You need to create a well crafted product and market that product until you can be the business that's paying out commissions to affiliates
Diversification is not just important, it's essential. A few years ago I relied heavily on my affiliate network to produce my income. Things went great for quite some time but then all the Google algorithm changes were implemented and I took a massive hit. It got to the point where it wasn't worth operating them and I sold them at a rather large discount just to wash my hands of them. I now have a service based business and I'm running into the same thing. I am relying heavily on two or three customers who make up a large percentage of my revenue. I've been pitching other potential clients, building new tools and hoping to correct this so I don't end up panicing if one of my customers decides to move on or put a hold on their spending.