That does you no good if you get nabbed with your phone in the decrypted but locked by fingerprint state. (which would be the common state since the password is a pain in the ass to enter if it's strong, so you do it on boot or something). So provided whoever has your phone can force you to put your thumb on your phone, forge your print, or cut off your thumb, they get your data.
Or fingerprint you at the detention center after you're arrested filming some cops beating a protester. Probably can just print it out to film and press it into the reader and open it. The CCC lifted the fingerprint of German Secretary of the Interior Wolfgang Schäuble from a glass he used at a panel discussion to prove how worthless fingerprints are for authentication.
