It's stupid or lazy to not use it IF it is functioning properly. Obviously your device isn't working...And your second device isn't working either. So get it replaced again. And yeah, 8.5 times out of 10 it works perfectly. The other 1.5 times I have to hit it a second time. I think that's pretty awesome for a hi-tech first gen feature. AND...Apple has already said that an update is coming very soon to address some issues. So im assuming it will even get better. This is totally expected with a feature like this. And I think Apple did an awesome job with it out of the box.
And as far as being stupid or lazy for not using some type of security on your phone...IT IS! why wouldn't you? And even if the FP scanner doesn't work, just go back to punching in a 4 digit passcode. Is that too hard? Then your phone is secure. I cant think of one other reason why anyone wouldn't use a passcode....Other then it being a PIA to punch in 4 digits every time...Aka lazy!
Guess mine is broken, not worth turning in again and if 85% is the best it can do when it IS working, I and many others still pass!