It's all a memory game I believe.
If a mobile provides huge internal memory, then no one needs SD Card. Well presently Z3 provides 8GB Internal memory while 25% gets used to OS. Left with around 6GB which does not seem good, as ACL and apps like whatsapp grabs up most of the space.
Now SD Card is used if one uses many apps/games or takes many pics or videos. So in this scenario, setting the default storage for SD Card is the best option for new apps and for camera media.
I suggest to have some feature where photos gets auto backup online while on Wi-Fi in the cloud and can be accessed from anywhere from the account. Once photos are backedup memory in the phone can be reused by deleting those photos.