"Diversification is a protection for ignorance, It makes very little sense for those who know what they're doing." - Warren Buffet
Don't know if this is Warren Buffet supporting the idea of diversification, or sarcasm. Let's take a look at what Warren has as of Dec 21, 2012.
1.) Wells Fargo & Co ........ financial
2.) Coca Cola Co .... consumer
3.) International Business Machines .... tech company
4.) American Express Co .... financial
5.) Procter & Gamble Co .... consumer goods
6.) Wal Mart Stores Inc .... department stores
7.) U S Bancorp Del ... financial
8.) Directv ... cable tv service
9.) Davita Inc ... health care
10.) Phillips 66 ... oil and gas
11) KFT ... food
12) INTC .... tech
13) Viacom ... cable tv service
14) Ingersol Rand ... industrial goods
15) National Oil Well Varco ... oil and gas
16) Johnson and Johnson ... drug
etc.
His personal wealth is comparable to the market value of a lot of those companies. I'd say he diversifies partly because he's forced to. It's hard to buy a lot of shares even when you have the money, and when you try it's hard not to end up buying more expensive shares. Then it's also not easy to find a nice way to cash out when you hold lots of stock.