My understanding (a nebulous statement on its own) is that bond funds held for the average duration of the bonds held have roughly the same "nav stability" as a bond of the same duration held separately. Rising interest rates also create a rise in dividend yield as new bonds are added.
Hence bond funds held for long periods of time (3 years for short, 10 years for intermediate, 20-30 for long term) should produce comparable "price protection" to owning the same duration bonds, and the yields will slowly ebb and flow with interest rates.
So owning bonds directly makes sense if you put enough dough into them to diversify and rates are good. Buying a bond fund is a good idea if you dont have the money to diversify, you think rates will rise, and you dont intend on selling for the length of the funds average duration as protection from short term NAV dents due to daily bond fluctuation. Oh yeah, and if you dont mind losing a quarter of a percent of dividend yield for the convenience.