The rebalancing is the key. I went back and looked at returns since 1990. I can't post the graph from work (blocked from photo sharing sites), it is pretty cool. Here is the overall result from 1/1/1990 to 3/1/2016 assuming you rebalance quarterly and using the S&P 500 and the Barclay's bond index. The Balance value is percent stocks/percent bonds.
Balance 26 Year Yield
The more stocks you have, the higher the variability. In the heady days of the dot com bubble (1999), you'd have been kicking yourself for having any bonds at all, but by January of 2003, stocks were the worst performing of all these combinations. By late 2007, the all stock portfolio was once again the top performer (by a hair), but by the end of 2008, it was the worst again by a decent margin. It didn't move out of the cellar position until 2013 and is now in 3rd place.
If you do a fit to an exponential curve, here are the R^2 values - a good projection of volatility.
You can see that you are getting a lot more volatility for pretty much no additional long term return. Of course, someone could show you a chart of one of the great stock runs and say you should be entirely in stocks. Someone else could look at the crash of 2008 and say you'd have done better since 1990 if you'd owned entirely bonds, but my big takeaway is I can get pretty much the same returns with less risk with a mix and rebalancing.
I'll try to post the chart when I get home.