Part of the allure is marketing, certainly affected some of my family members.
By calling it the iPhone 6 "Plus", and only charging $100 more, it drives many people to buy it instead of the normal iPhone 6 because "it must be substantially better", even though (I suspect) some may regret the size, coming from 3.5-4" handsets.
Being realistic, you're paying for the screen, a little larger battery, and OIS -- not like one runs an A7 and one an A8, or one is made of plastic, etc.
That said, 6 Plus 64GB on the way for me, I like larger phones, but even I was taken back by the height of the phone compared to even the Note 3, LG G3, etc.