Bonds: When, Why, Which Ones, and How Much?

I started working full-time in 2005 and ever since then have been contributing a minimum of 15% of my pre-tax income to 401/403/etc tax-deferred retirement accounts.

My investment approach between then and now in these accounts has been very simple: Buy and hold VSMPX and VTPSX at a ratio between 80:20 and 70:30.

My rationale for this approach, and overall investment philosophy has been: I am relatively young and will not be retiring for a long time. What I am trying to do is invest broadly in the world's productive and innovative capacity as a whole.

I am now in my early 40s and am wondering if I should be starting to include a relatively small amount of bonds in my portfolio sometime in the near future. My time horizon for needing to withdraw/spend any of the money in these retirement accounts is still quite long, but not as long as it used to be.

I have a vague notion that at some point a person should start hedging (if that's even the right word) against the risk of medium-to-long term losses/underperformance in the stock market by adding bonds to their portfolio, because bond prices have historically not been tightly correlated with stock prices. But I don't have any idea when and to what extent a person should start doing this.

I also don't really understand what the underlying philosophy or value proposition is of investing in bonds, as opposed to the philosophy I outlined above of investing broadly in stocks.

I would be interested to hear any input that anyone has on this.