Diversification is a means to an end. It is not the end.

And it’s not always the best means to achieve the end, which is portfolio risk management.

Sam Lee discusses The Trade-Off Between Diversification and Knowledge (And when to make big bets) in a July 24, 2014 article for Morningstar:

Diversification is often treated as an unalloyed good. It’s not…Warren Buffett said something as perfect as can be about the subject:  Diversification is protection against ignorance.