Seems like it would be useful in an environment like a gun range where you aren't relying on it for safety.
A) as another poster noted, the whole reason you go to a gun range is to get more better at shooting the guns you have, so that if you need to (or want to) use them for real later - either quickly like self defense, or more methodically like hunting - you know how well you can aim with them, what realistic distances are, how much kick to absorb or correct for...
B) Which leads us to a fingerprint scanner being a disaster in a crisis situation like a home invasion, you don't have the time for that nor want to rely that a gun you might have not touched for a while still has power enough to enable the fingerprint scanner. Similarily if you go hunting, it would REALLY REALLY SUCK to travel for hours to find out your fingerprint friend has no power or just decides that environmental conditions mean your fingers are now invalid.
So said fingerprint scanner gun would never be a gun you would use in real life, making it pointless to shoot at the range,