My phone is my everything. It is my newspaper, my communication device, my shopping list, my map book, my fabulous public transit scheduler, my emergency device, my flashlight, my whistle and a lot of other things that are not as useful while out and about still drain battery life ( camera, PDF generator, computer hard drive, document reader and editor etc...)
As I mentioned earlier I think it is rude to shove close to someone without excusing yourself and apologising for any inconvenience but I don't think it's unreasonable to keep as phone charged even if you are not a doctor.
For example, if I was taking public transit home late at night, I want my phone to tell me when the buses are coming and which one to catch and to distract me during the rude and to make me feel safer during the ride and the walk home.