Glenn Greenberg's not telling you to pay close attention. Nor is he suggesting you should pasteurize your fruit drinks. Greenberg is saying he disagrees with the oft-used expression "Don't put all your eggs in one basket" when it comes to investing.
Since 1984, Greenberg has outperformed the S&P 500 by almost 10% per year. One of the chief reasons he attributes to this is a strategy of anti-diversification! For example, he will not buy a stock unless he is willing to commit 5% of his portfolio to it.
This forces him to know all the companies he buys very well. He attends their meetings, reads trade journals, and knows his companies better than the sell-side analysts. Sounds like a pot-shot at sell-side analysts; how does he know how much they know? Well Greenberg started out as one in 1973.
Following a brief elementary and highschool teaching career, Greenberg completed his MBA at Columbia and joined JP Morgan as an analyst. Greenberg saw first-hand the follies of the industry, as his firm could not buy enough Avon or Polaroid shares, despite the fact that they were trading at over 100 times earnings!
Needless to say, the Dow Jones declined 40% over the next two years. These events laid the foundations for Greenberg to be skeptical of prevailing wisdom and to make sure he understands a company extremely before he buys an individual stock. Keeping his portfolio at between 8-10 companies allows him to do just that.