Many smartphones and programs are using fingerprint's to secure the phone or encrypt data. In many cases, those programs want a PIN as backup authentification method.
A PIN just got 10000 possible combinations, while a fingerprint got many more. So an encryption isn't safe if there is a backup pin, is it?
The same question for a phone lock-screen: If there's a 4-digit PIN, people could just bruteforce this pin instead of bruteforcing million of fingerprints, couldn't they?