I have thought, in the context of, say there's a market boom, and for whatever reason I decide its not a good time to be stocks (lets just skip over the details of that for now), and I've paid off all my high interest debt (and in a bubble - there's a good chance all debt will be high interest debt)
Then I think it would be a good time to buy bonds - as others have said, after a bubble, interest rates drop, which means bonds gain value, and then you can sell them and buy shares cheap. But at the moment, even if there's a bubble, which is contentious and in any case I doubt there is one now, how are interest rates going to get cheaper than they are now?
Personally, I'm waiting for the first of the deflationary economies (Japan/EU) to pull the trigger on helicopter money - and IMHO this age of ultra-low interest rates will pass.