It just means that when something goes wrong, such as a forgotten password or a botched update, their data that would have otherwise been recoverable is now lost forever.
I'm not sure I know anyone who's had a computer stolen, but I know lots of people who have lost data.
Edit: I do know one person who had a computer stolen. It was a work laptop while they were in SF, and I'll concede that FDE probably does make more sense on a work-related computer. I was only arguing that it's more of a hindrance on personal devices that mostly stay in the owners home.