I had been told by numerous financial advisors during my financial infancy to "diversify, diversify, diversify."
Diversification is a tool for preserving wealth, or growing assets slowly and conservatively, but not for building great wealth. If you consider the list of richest people in the world, most did not attain their financial status through diversification.
It is also in the interest of brokers to advise diversification, because it provides them an earnings stream for transferring your current holdings into other assets.
Being a fiscal conservative, I started diversifying my assets when my net worth reached $1.6 million. Had I not ever diversified, I would be worth north of $40 million today (before taxes). But had things turned against me, I could also have gone broke - I saw this happen to some of our competitors. So know what you're doing before you diversify!
My advice is to develop your own financial intelligence. I definitely know a lot more know than I did 5 yrs ago. I have worked with many brokers over the course of the years at many institutions. In many cases, I now trust my own judgment vs a financial planner's or advisor's advice. Financial intelligence also allows you to filter through the multitude of advice you receive.
In the old boom days of the stock market 1999-2000, the brokers were useful mostly for getting access to IPO stock at pre-IPO prices, and the deck was heavily stacked against retail investors. Brokers would offer their top clients shares at pre-IPO prices and flip them within a few hours of the start of trading, for sometimes obscene gains, all at virtually no risk to the client. Since those wild IPO days are gone, the full-service brokers have lost a big reason for using them.
I have also learned that you need look into your holdings in more detail before you consider yourself diversified. Many funds have similar holdings. Even if you are invested in small-cap, mid-cap, large-cap, and international funds, this may not prove to be adequate diversification. During the 2000-2002 mini-recession, foreign (European) stocks were sold off just as heavily as US stocks. The world economy is so co-dependent now, that a recession in the US would trigger a world-wide recession, because much of world-wide spending is driven by the US.