For the early retirement investor there's no good reason for a large slug of your investments to be in bonds. That goes doubly so at current yields, as yields are the biggest predictor of what kind of return you'll get in bonds. If you need bond appreciation for your bond investment to be worthwhile, then you're just speculating and you may as well be in stocks.
It's been shown historically that stocks have the best return on investment. Especially over the long haul. Which, as early retirement investors, is us. And that's the end of the story. Anybody telling you that bonds are a good investment after that, are just wrapping themselves up in a warm blanket because they don't like the scary stock market. And if that's you as well, then great. Buy your bonds "so you can sleep at night." But here's the thing, your final investment allocation is going to look a lot like mine minus the bonds. That is, if you decide you need a million dollars to retire with your 30% bonds, you'll still be 70% stocks ($700,000). My all stock retirement is going to basically be the same, but 100% stocks and $700,000. You'll have definitely worked longer to get that nest egg. I'll have worked less, BUT maybe things don't work out for me and down the line I'll have to grab some income from somewhere. What's the difference? The bond investor DEFINITELY works longer. Me? MAYBE work the same as the bond investor.
So, if you want to work longer now to have your warm blanket, go ahead and build up your bond allocation. If, on the other hand you want to retire sooner with the realization that down the line you might have to be flexible about working here and there then forget the bonds and just buy stocks. And that's really what these arguments come down to: for some people, the idea of going back to work after retiring is HORRIBLE. It has absolutely nothing to do with the merits of owning stocks versus bonds. Because as an investment in and of themselves bonds are qualitatively worse than stocks. So, if the idea of working here and there in your retirement sounds horrific to you, you already know: you're a bond investor.