Let's just say you have $1M in an IRA and 10 years til retirement. The market tanks 30%. In the following 10 years, the value doubles (roughly 7% return).
Converting:
I convert, and pay appx 40% in taxes taking it out of my holdings. I now have about $400k left of my $1M. It doubles to $800k. I pull out 5% per year ($40k) tax free until I die.
Not-converting:
The $1M goes to $2M. I take out 5% per year ($100k) and pay 30% tax, netting $70k until I die. That's a hell of a lot more.
Yes you would pay less to convert the lower amount than if the market didn't tank, but you would be better off in this scenario NOT converting.
My point is just it depends a LOT on how much you have and what tax bracket it throws you into, as well as how much time you have left. Opposite extreme, my son has probably $50k and is 30. For him it probably makes a lot of sense. For those here that aren't super confident about their financial jiu-jitsu, please consult a financial advisor for help.