You are disposing of a drive and want to ensure the data is not leaked.
If the data was stored in plain-text (i.e. not encrypted), you would have to ensure the data was 'shredded'. Just formatting the drive would not be sufficient; formatting doesn't overwrite the data, which is why there are 'unformat' utilities.
Instead, you'd consider a tool like DBAN. There are various erasure standards and as @Raystafarian points out, you can be all but sure the data is unrecoverable if you do a 7-pass shred. In truth, there are no reports of successful recovery after even a single pass on a modern mechanical hard drive, but it often pays to be conservative.
Matters are more complicated when considering SSDs. See this article (thanks to @Raystafarian for the link) for more information. SSDs remap blocks, which means you can overwrite all the data on the drive and some of the old data may have been remapped and not actually overwritten. DBAN doesn't handle SSDs.
Okay, but this isn't what you are doing. You have stored the data protected using Bitlocker and aren't planning on shredding or even overwriting the data at all. What you are suggesting is actually similar to how SSDs often implement 'secure erase'. They encrypt all data and then, during the 'secure erase', simply overwrite the block containing the key. Without the key, there's no practical way to recover the data.
This is true in your case, too. You cannot recover the data from a Bitlocker-encrypted drive without knowing the key, assuming no vulnerabilities in the Bitlocker implementation or in the underlying encryption algorithm. The fact that you know a great deal of the plain-text ("known Windows files or known data structures") is irrelevant; the encryption algorithm is strong despite known plaintexts.
In this case, against most adversaries, I'd suggest just doing a complete (as opposed to a quick) format. Even that shouldn't be necessary, but it's probably a good plan. Against a well-funded government or evil criminal empire? Well, you probably have other problems. And then I'd definitely be wiping the drive (using DBAN or a similar SSD-capable tool) and physically destroying the drive.
Best Answer
It's generally useful. If you have automatic unlock, some systems might allow a determined attacker to bypass it, but it would take significantly more skill than just booting up a USB stick. However, you should have a BitLocker PIN to actually lock it down.
Yes, but they cannot read it, because the hard drive is encrypted.
BitLocker on new laptops uses the TPM chip to implement automatic unlock. One key is stored in the TPM such that it can be read back only if the system is booting in exactly the same way – same firmware settings, same PCI hardware, same boot device, same digitally-signed BOOTMGR.
(This means that your login password is not used for unlocking the disk; at the time you're staring at the login screen, the disk is already unlocked, so that the OS could be loaded from it. Everything on the system partition, including the OS itself – except for BOOTMGR – is encrypted.)
If you try to boot from a USB stick (or make any other changes, e.g. disable Secure Boot signature verification) the system disk will not automatically unlock anymore, even if it's still the same laptop; the only way to access it is by knowing the recovery key.
So the thief cannot obtain your Windows password hash (which would indeed be fairly easy to bruteforce) and they're limited to poking at the actual Windows login screen, or performing some sort of hardware attack (e.g. "cold boot" & reading the BitLocker key out of RAM).
Note: If the laptop has a discrete TPM chip (as opposed to fTPM), they can fairly easily intercept the actual signals between the TPM and the CPU and find out the BitLocker key this way. I believe BitLocker's "TPM + PIN" mode guards against this, because the PIN is required by the TPM to reveal the key (and the TPM itself has a lockout mechanism).
Finally, BitLocker won't prevent the laptop from being wiped or reused. The TPM is by design always possible to clear and reinitialize by anyone who can access the firmware settings screen. (Unless the manufacturer also stores the firmware password in the TPM, like HP does... but apparently it doesn't stop people from unlocking laptops by replacing the whole chip.)