I think you've mixed that one up, its usually something like "date the body, marry the glass", implying you'll buy and sell many camera bodies over time but keep good lenses. For non professional work you'd want to own both the body and lens, going to the rental shop every time you go on holiday or to a birthday party would be a pain.

For professional work you rent both the lens and the body so you can use the highest end gear specialized for the one particular job and aren't left hanging on to a $10k lens you use once a year.