You know, it's because of people like you (and a vast mass of people who don't care enough) that we can't have long-working phones. In short, a phone with greater battery life is a boon when you:
* travel a lot. It doesn't have to be often - but when you leave your room early in the morning for a long day in a new town, and use it a lot, it really gives you peace of mind to know that you will be able to make that last important call in the evening.
* forget to plug it in. Seriously: do I really really have to plug this gadget in evry single evening? what if I forget or don't have the means to, every once in a while? It's a flagship device we're talking about; isn't it supposed to make my life easier?
* want your phone to have extra margin of reliability just in case - either to prolong its life when the battery deteriorates over time, or use it in cold weather - I am not sure if it's permanent damage, but battery holds less charge in the cold.
The iPhone is not the worst offender - it's probably slightly above average as far as smartphone battery life goes, but, to me, saying that battery life is not an issue sounds like a blasphemy: we (the consumers) have just squeezed barely adequate battery life from the manufacturers and voicing the opinion that it's enough feels like inviting back the compromises that they (the manufacturers) are far too happy to use.
The battery pack sure solves some woes, but it's not by coincidence we call it a mobile phone: it is supposed to be self-sufficient. Do you like to carry around a second box on your body at all times, just to be able to use a smartphone reliably? I don't. Battery pack should be a bonus that's nice to have, not a requirement to last through the day.