The vast majority of bond funds do not offer downside protection at the moment.   Depending on what happens, they will languish and lag the real rate of interest on assets generally (especially housing) by a huge gap, or, if rates start going up, they could tank.  Equity-like behaviour in bonds is very undesirable and people could be scorched.

If you're talking about individual bonds, you need to get a handle on the credit risk and it might be worth consulting a pro/advisor to do this, depending on your level of knowledge.

High-yield bonds offer something different and can be beneficial for diversification, but you really need to know what you're doing and need to understand the instrument.   They usually behave like hybrid securities.   

Short-term funds and money market funds can work, however, they are not yielding much more (if anything) than an amped up savings account.

10% or less of a portfolio is usually acceptable for someone who is still of working age.