I know when I move to the F fund I get a lot of of grief from readers. They tell how it's the worst performing fund for years, you take on risk when the G fund is more reliable, etc. But of course I am not investing in the F fund. I use the F fund in lieu of G when I am on the sidelines and the AGG technical picture looks promising.
So yes, we shouldn't invest in the F fund (except maybe a small percentage as part of a buy and hold strategy) but there are times to take the chance and trust it. But I think I know why they named the bond fund the "F" fund... Fill in your own joke here.