Bitlocker doesn't support USB key + PIN (password). The reasoning was "the password is too weak to provide additional security". Isn't that flawed? With a USB key and password, I've got some level of two-factor, right? The USB drive contains 128/256 bits of key material, and the user could provide as large a passphrase as needed. That way, stealing a USB key doesn't grant access, nor does somehow finding the password (watching, listening, etc.)
The same for TPM. I'd like to use the TPM to verify the startup environment. But I don't want to trust my entire key to chips that may be very poorly implemented. (I recall some news of attackers being able to just open up the TPM and access the key material directly.) If Bitlocker used my password, I could get the best of both worlds.