Yeah but you care less about your investments if you include the bong asset class.
Last time I looked, having less than 20% bonds in your portfolio increased volatility without increasing returns, while having less than 20% equities reduced returns without reducing volatility.
So somewhere between 20/80 and 80/20 is the sweet spot.
Of course, the types of equity and bond make a big difference. Look at Wellesley with a 35/65 mix of large cap value and intermediate bonds...has similar long term return rates to the S&P500 and other all or high equity funds...
Besides bond terms, credit quality and interest rate sensitivity, each type of bond 'moves' quite differently from others...high yield, muni, treasury, financial, etc. Good bond asset allocation is just like good stock asset allocation...some are more appropriate for accumulators, some more for those already in retirement.
Several pundits suggest that the only bonds you ever need to own is something akin to vanguards short term investment grade fund. I dont necessarily disagree with them, although I also own some GNMA, High Yield and until recently some california munis.