Yeah we often use each others' phones. Just for practical and convenience reasons. I can't recall that we would have used each others phones pre-kids but we do find it useful nowadays. I imagine there will come a time where we don't do it anymore, although it probably won't be a conscious choice. Just a natural phasing out. Or maybe not, who knows.
We both share our locations which is useful for very boring reasons, like if DH is trying to catch up with me and the kids after work but I don't answer my phone (because I am notoriously shit at answering my phone). Or if he's been travelling for work and I'm checking how far away he is so I can get dinner on. We also like the security of having that information in case of an emergency, especially if one of us is out with the children.
None of our kids have mobiles, so it's not unusual for one of them to borrow either my phone or DH's to play games etc. In which case that parent might borrow the others phone to check footy results/search a recipe/check the calendar/use the camera. Sometimes we might look through each others photos to see if they have any new pictures of the kids.
I suppose technically this means we have access to each others messages, emails, WhatsApp etc, but I definitely wouldn't check DH's. I doubt very much that he checks mine. I suspect he would feel the same as I do about that; that I have nothing to hide, but that it would feel like a bit of a slap in the face to be "checked up on" when neither of us has ever given the other any reason not to trust.