It looks like Bulletshares are only available for higher risk categories, like junk bonds ("high yield") and corporate bonds (lower quality, but still investment grade).
http://portfolios.morningstar.com/fund/summary?t=BSCM®ion=usa&culture=en_USIf someone follows this advice and uses junk bonds, they will be very unhappy when a stock market correction makes a severe dent in both their stock and bond allocations. Junk bonds tend to collapse alongside stocks, making them not that useful for diversification. Government bonds are much better, surviving crisis like 2008 far better than junk bonds did.
I would encourage everyone to have some allocation to bonds. Even if you want 100% stocks... when will you learn to rebalance? When will you develop good behaviors surrounding your investments? Most likely you won't, while having 10% bonds will provide the opportunity to rebalance. It becomes more important the closer you get to retirement.
Note if you have enough to retire, and you risk it anyways, you can get burned by risk. Risk isn't just for growing your portfolio, it's also the stock market crashing just before you planned to retire. And suddenly, you don't have enough to retire.
You should also read about "sequence of returns" risk before believing you can retire on 100% stocks. There's a risk your withdrawal rate becomes much higher after a crash, and could impact your retirement.