Rene Ritchie for iMore:
Battery life is the currency of mobile devices. Every feature you add, you pay for in battery life. That includes screens, radios, and it includes lightness. If a company like Apple is willing to pay that price, it shows how important it is to them — or how important they believe it to be for their customers.
A thicker, heavier phone starts to become a drawback when you try to hold it up for prolonged periods of time. It causes fatigue and eventually prevents you from using it, charged or not, for as long as you'd really like to.
An iPhone 6 or iPhone 6 Plus with the same thickness as an iPhone 4 would be too heavy for many people to read iBooks or watch movies for long periods of time, for example, while in bed or while on a flight. It would also be harder to balance and use one handed while walking around.
It's the same reason Apple's been striving to make the iPad ever thinner — to make it ever lighter and more usable.
That's one argument, a better argument might be that battery life is the least important metric in mobile devices, especially when it comes Apple, because when it comes to hardware/ID — the actual currency is design — witness the oohs and ahhs at every keynote over millimeter reductions in device thickness versus the slow cap when it's announced that said device will have similar battery life as its predecessor. Put another way, the design of a new iPhone never starts as a conversation about battery life, it's about direction, specifications, manufacturing processes, and keeping fingers crossed that somewhere along the way software and circuitry improvements will make up for any reduction in physical battery size. Because he's a nerd, Ritchie's bias is to assume these decisions are tied to usability, because I'm a cynic, I think it's far more likely they're tied to visually eliciting visceral, emotional reactions from consumers, whom having never even lifted one, happily pre-order one for themselves.