Sure, it doesn't stop a criminal, but it implies privacy that could be held up in court against unlawful search.

I have my phone set up to enter a long code on boot (which goes to LUKS) but the lockscreen PIN is much smaller. The low entropy on the lockscreen doesn't matter so much as it is capable of restricting the number of tries, delaying after a certain number of failures, etc.

I agree, I don't see why it'd be unreasonable to type a passphrase on boot.