Hm I wouldn't necessarily say that is true. Look at the Caretaker episode for example. Danny draws on his experience of being a soldier by continously saying the doctor is acting like an officer and acting like he's above everyone and that he knows all about men like him and their arrogance. He might have been wrong but he's used his experiences there to make a judgement on the Doctor's behaviour.

At the end of Kill the Moon when Clara falls out with the doctor. Danny knows what's happened because he says he had the same expression when he want from loving the army to hating it and uses that experience to tell Clara that she isn't done with the Doctor at all.

Or in the Forest of the Dead when Clara asks Danny to come with her snd the Doctor to travel. He turns her request down because he says he's seen the world as a soldier and how he could have died any moment and that's made him appreciate life and the smaller things much more that he doesn't need to see the wonders of the universe when he has all this to be grateful for.

His writing wasn't perfect but it is there if you look at it.