There is a slight dichotomy here. Successful investors may tell you that most of their money was made in just a few stocks. That is absolutely true! But to survive long enough in the markets to make money you need to ensure that your risk is diligently managed. That is where diversification comes in. Too much concentration can destroy your equity portfolio and hence your risk needs to be constantly monitored. Irrespective of whether you are a trader or an investor, one of your key goals is to preserve capital and that can only be done by diversifying your risk.
Imagine if you had a portfolio of infrastructure stocks in 2007 then you would be sitting on 60% losses till date. That kind of concentration does not do you any good. The best of investor diversify and you must understand that when you buy a diversified company that itself is diversification. Also when you reject 100 companies before buying the 101st company then that is also a form of diversification.