I thought the entire point of full disk encryption was to keep your data safe even if the computer is stolen. If FDE is not effective against that, then why bother? It's not like FDE protects you from online attacks so if it can't protect you from physical theft, what can it protect you from?
It's not like a deadbolt prevents your house from being broken into, or a locked door on your car, but it's a lot better than nothing.
Drive encryption likely stops the casual thief, but it's not perfect. And like the XKCD comic, if they REALLY want the contents of your drive, they're going to use a $5 wrench as the master key.