The point is that the PC boots up fully unattended. People could access my data without a password if they stole my machine and somehow managed to log in to it,

The point is that the PC boots up fully unattended. People could access my data without a password if they stole my machine and somehow managed to log in to it, say through a bug in the desktop manager on the login process. The LUKS key is in memory, so I guess if they can somehow read that, they could also access the drive. But I figure the convenience is worth it, I don't expect to be a high-level target for this. And if I were, the 5$ wrench attack would probably work better. If they stole the hard drive, they wouldn't be able to access the data, the TPM wouldn't release the key.