I wouldn't. There are times my phone doesn't 'like' my finger and I need a PIN to unlock it. Additionally - new phones are shipping with encryption enabled, in order to unlock the encryption initially after a reboot you need to enter a PIN, a fingerprint isn't good enough.
Also - in some cases law enforcement can't ask you to provide a PIN as doing so would violate your 5th amendment rights (in the US) but asking to provide a fingerprint to unlock a device HAS been allowed so using the feature to secure your phone prior to LE interaction is MORE secure by nature of having a PIN.
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