The thing about finger prints is that they are really really really hard to change ... you can change your pin/gesture n times a day and not care.

But more often you either don't change them many times a day (seriously, how many do that?) or don't have PIN at all. Peeking someone's PIN code and snooping around the phone while the owner is away is much much easier than reconstructing the fingerprint well enough to pass the TouchID. It was said more than once: TouhcID is not perfect, but better than nothing, and before TouchID "nothing" was more likely, because all of the hassle with PIN.

Sure. However, in practice, what difference does that make?

In many cases fingerprints are perfectly fine to use. However, they do have glaring problems as well. So they are anything but perfect, I’m just not sure what difference that makes to how they are used in practice right now.

True.

On the other hand, how often do people use this?

I, for one, don't lock my phone at all. And most people I know don't change their PIN/gesture ever.