From a Barron's interview with Marty Whitman:
On how important modern portfolio theory is to him:
"...as far as value investing, control investing, distress investing and credit analysis is concerned, that stuff is absolute garbage."
On the notion that portfolio diversification is important for downside protection:
"Again, in terms of value investing, control investing – though we are not control investors; we are a mutual fund—distress investing, credit analysis, this stuff is worse than useless."
Then later in the interview Whitman said:
"...diversification is a surrogate—and a damn poor surrogate—for knowledge, elements of control [of a company] and price-consciousness. If you are really a value investor and do deep research, how many investments can you be involved in at the same time? If you are a high-frequency trader, you could trade 100 securities today. The real value investors are lucky if they can do 10 investments at a time."
"Absolute garbage" and "worse than useless".