BitLocker in TPM-only mode protects against modifications to the boot process (such as Live CDs, etc.) while also providing reasonable (hardly perfect, but reasonable) protection against physical attackers (you'd have to freeze the RAM from the live system after bootup but before login). This only works because it's expected that the attacker is stymied at the login screen. By providing a way to bypass login with arbitrary permissions, you are reducing the security of any system using BitLocker.
Otherwise, it is a valid point.