For an investor many crucial differences exist between stocks and bonds. Stocks typically have higher returns while bonds have more security. Stocks dividends are usually partially tax deductable for an individual investor and completely tax deductable for a corporation. The lower returns of the bond may well be worth the reduced risk that an individual investor is under although the gains in terms of interest in zero-coupon bonds or any coupon bonds are taxed at the individuals current marginal tax rate.
Controversial so called "junk bonds" provide high returns in exchange for more risk of default. Millions of dollars have been made buying these bonds with low S&P ratings and achieving higher returns. The question to ask would be if these higher returns compensate the investor appropriately for their risk. Many bonds have a built in feature of corporations not being allowed to default on payments unless they go bankrupt. This causes the bonds to have less risk then equity in the same firm. Bond holders also only have the opportunity to achieve limited gains as dictated by the bond contracts coupon rate, while shareholders have the opportunity to make unlimited gains.
I beleive that in a well balanced portfolio and individual should include both stocks and bonds. If the risk side of my portfolio consisted of stocks then I would balance that out with lower risk bonds. Meanwhile if I was participating in the higher risk junk bond market I would balance that out with stocks that have lower risks. It is my belief that a strong portfolio should consist of approximately 10% risky equity, 40% moderate equity, 20% low risk equity, 20% of varying degrees of bonds and 10% of risk-free's such as treasury bills (T-bills).
The whole portfolio diversification is dependent on an individuals risk tolerance though. I am have been working on a financial theory of mine which would produce maximum returns over years while minimizing riks. If successful my investment portfolio would drastically change to hold the mixture of securities I beleive I can create that would achieve above most fund managers returns. Fund managers who tell you they can acheive above average returns are lying and you should not beleive them. The best you can do is earn what the market earns, any attempt to earn more then the market can just as easily provide you with low or negative returns. More on this topic in the future.