I do think it could be planned better than that. If I wanted to gain access to someone's email, Facebook or buy things on iTunes, etc. here's what could be done - would be easy for a work colleague/spouse/etc.:
1. Get the fingerprint (I'm guessing it's going to be the thumb for 90% of people) from a glass or something else.
2. Prepare the fake print (taking all the time you need)
3. Then at an opportune moment (going to the restroom at a restaurant, a meeting at work, at a bar, etc.)
4. Then quickly grab the phone - get what you need in a matter of seconds - then put the phone back.
I'm an Apple fan, and maybe there's a flaw in what I've outlined... but it would be good to know whether or not this could be done in a relatively easy way. Of course there has to be motive and effort... but a scheming spouse, work colleague, etc. could do it and you wouldn't even know!! It would be hard to find out also.
I will be using it - I generally never leave my phone unattended (except for charging at night). But I would like to know if it's possible this way.