Even trying to plan everything in advance, it would be incredibly difficult to navigate an unfamiliar city by myself, as I was, with only a paper map and a route drawn on it. I remember on our family vacations driving across the country that we always needed a second person as a navigator, and even then there was often a lot of guesswork and missed turns.
My phone, on the other hand, would verbally direct me, and do so even better than a human navigator could. I would only have to occasionally glance at the phone, most of the time simply listening to the audio cues. If you make a wrong turn, the directions update on the fly to get you to where you need to go. Getting lost is damn near impossible.
This is one example where a smartphone not only replaces an older technology, but is probably an order of magnitude more practical. There's simply no comparison to using paper maps. Being disorganized or not has nothing to do with it... it's actually solved a real-world problem for which paper maps were really only a partial solution. Yeah, I'll still call that "indispensable." Maybe not in a completely literal sense, but close enough for all practical purposes.