If stocks historically have higher returns than bonds, then selling stocks to buy bonds reduces portfolio returns over the long-term.  Conversely, for those with the fortitude to sell bonds and buy stocks when there’s blood in the streets, the process reverses.  The effect is more likely reduced volatility than increased returns.

I’d rather build a portfolio by planning for liquidity needs rather than putting the client’s emotional IQ in control of his asset mix by managing to control volatility.  Warren Buffett doesn’t rebalance.  I prefer to manage risk in the context of valuation.  That, I can control.