But I usually don't leave my finger with my phone if I forget it in a bar, or in case of theft.

The lock codes are far from secure when used in public, people can guess pretty fast what is your code by sneaking over your shoulder in the subway, by roughly seeing the moves of your fingers even in front of you, using the reflection of the windows in the bus, or just follow the smudge on your screen if you are using a geometric pattern. To be fair, they probably don't care at all about the code and the data anyway, they just want the phone to sell it, but I still feel better if I know access to my data is not particularly easy.

Now, if someone threatens me and kindly asks me to unlock the phone for him/her with a knife on my belly, I'll happily unlock it with my fingerprint and even give away the code to unlock without fingerprints. Hopefully he/she will leave me with all my fingers. But if I lose my phone or just find a pickpocket, I don't think they would come back and track me down to get my thumbs, and I'll be glad that there is no smudge to follow on the screen or any code to spy before stealing the phone.

Has any pickpocket ever tried to unlock a phone with a fuzzy fingerprint left on a doorknob or on the screen? The guy achieving that would definitely deserve full access to my phone. I would be more concerned about what else that guy would be able to do with my fingerprints if he managed to make copies good enough to unlock the phone.

By the way Juiceme, which procedure did you use to enable double-tap-to-unlock on Sailfish X? I haven't read the corresponding thread on TJC in a long time but I thought enabling it still had some downsides. I am using an Xperia X Compact, I think some sensors are not enabled, maybe that would be an extra issue.