If you want some fixed income investment, should you buy bonds directly, or should you own a bond fund instead?
The answer depends on the matter of cost, and convenience.
First, the nature of bonds are somewhat different than a bond fund. The maturity of a bond decreases year by year, while a bond fund's maturity will remain relatively the same. If you are sure you need some money five years from now, you can buy a five year US Treasuries Note, and you are guaranteed the principal back at the end of five year. If you had purchased a bond fund, the principal at the end of five year would depend on interest rate differential.
Owning a bond fund is more convenient. You can sell shares with a phone call. You get check writing capabilities. You also get income distributed monthly.
Owning US Treasuries in Treasury Direct costs less than the expenses of a bond fund. However, the tender forms and other forms used to manage a Treasury Direct account is less friendly than a mutual fund's customer service department.
Lastly I believe in cost wise, a mutual fund should be pretty competitive in the area of corporate bonds, or municipal bonds. There is no such thing as a Treasury Direct equivalent program for the municipal bonds for example.