During the Zimmerman trial, the prosecution argued that certain of Trayvon's texts were inadmissible, because it could not be proved that Trayvon sent them, as opposed to someone having access to his phone.
If your phone unlocks by fingerprint, and the phone maintains a log of access and whether it was by fingerprint or passcode, then a prosecutor could successfully argue that you, beyond shadow of doubt, were the person on the phone when a call/text/pic was made, as opposed to some other person working your phone without your knowledge.
Apple, like many companies, uses data from your iPhone/iPad. Just how much Data they are logging, no one knows. And since Apple is a closed system, (as opposed to open source) you have to accept their word for it.
So what?