0. The police ask for ID, you provide it, and they search your now unlocked phone without your consent?
1. Your phone battery dies and you need to provide ID.
2. A perfect digital static copy of it is created, say from an inadvertent screenshot. Now there's a risk of having that get in the hands of someone else.
3. What if you need to give your ID to someone? Now you potentially have to give your phone to them.
4. How are counterfeit DDLs going to be policed? It seems like it makes making fake IDs trivial.
While going DDL only seems like A Good Idea(tm) in theory, it's probably best used as a backup to a physical card rather than the other way around.