you probably answered your own question regarding asset classes.. I would hate to have large chunks of my portfolio wiped out by a stock market crash.
Often, this is when things like bond yields & gold prices rise which offsets some of the damage.
If you are investing in a SIPP for instance, you may be wise to gradually reduce the percentage invested in shares unless you are confident in your 'defensives'.
At the end of the day, everyones attitude to risk is different, so there are many points of view out there, I'm just expressing mine.