All Android versions I used don't support setting an encryption passphrase that's separate from the unlock pin/password. As a result one has to choose between a very long unlock password one has to enter all the time or a convenient short one making encryption basically useless. Given this trade-off, I think using a proper, long passphrase for encryption with a fingerprint for normal unlocks is preferable to the alternatives. It's particularly irritating that this restriction is not at all technical; the only justifications I've read were along the lines of "it might confuse users".
Less importantly, for my phone, a shoulder-surfing attack seems much more likely to me than someone specifically copying my fingerprint.