I've taken the last few days to actually play the game and figure this out for myself.
Can you play without going out often? NO
I'm talking about the average player, not someone who wants to buy a drone and strap their phone on it — or someone who is interested in spending loads of $$$ on in app purchases.
It became apparent that while yes you can sit stationary and Pokemon do show up, you simply cannot progress in the game without moving.
Pokestops give you Pokeballs. If you only catch Pokemon as they pop up and never move about and visit Pokestops, you can't get more Pokeballs. Your only option then is to buy them.
Incense are items that attract Pokemon to your location. You get 2 to begin with, and only get another when you start to level up. Once they're gone, your only option is to buy them.
Gyms, I've found, are not essential to leveling up and playing the core game. So while you won't be able to use them without visiting them it doesn't really matter (early game at least).
And finally, if you don't catch Pokemon you won't get XP. Which means you won't level up. Your only option is in app purchases.
Note: Eggs require walking, biking, or very slow driving to hatch. There's no way to get around this, so basically this part of the game is off limits to someone who doesn't get out.
I've been playing quite rigorously the last 96 hours, and according to my iPhone Pokemon Go has spent 6.4 hours on screen. My data usage, also according to to my iPhone, is 59MB. So for a whole month, if I continue playing as much as I do, I will use ~440MB. This seems fair, based on how much I've been playing.
It eats up battery like no other. That's all there is to it. I've taken to using a portable battery pack.