The major problem is the possibility of self-incrimination. Let's say a drugs user is raped; and unrelated to that, her phone is full of text messages back and forward with her dealer. Obviously, that person is going to be VERY wary (flat out unlikely) to hand over their phone to the police. Does that person just have to accept that they can't get justice for something as serious as sexual assault, because they don't want the authorities having free access to their devices?
It doesn't even have to be something like drugs... it could be simple references to basic things... like downloading movies off the internet... buying a Kodi box off your dodgy neighbour. Minor things that are realistically not going to cause problems, but would make many victims think twice about handing over a phone.
Not to mention just personal stuff... like intimate pictures, or even just text mesages between partners. It's not even about those being used as "evidence against the claim" - it's just that it's totally understandable for people to not want anyone - even a police officer - reading through their personal messages that are meant for close friends and family! There's literally nowt dodgy on my phone (it's all on my PC
) but I still wouldn't hand it over to anyone for them to read through text conversations between me and my wife. Just no.