I'm a storage whore and I really use my iPhone a lot so I have always purchased my iPhone's with the max available storage.
iPhone 3G (2008)> 16GB
iPhone 3GS (2009)> 32GB
iPhone 4 (2010)> 32GB (was really annoyed they didn't have 64GB that year)
iPhone 4S (2011)>64GB
iPhone 5s (2013)>64GB
iPhone 6 Plus (2014)>128GB (coming tomorrow)
I like having a large music collection instantly available which accounts for around 32GB of storage. I have over 300 apps (do I use all of them all of the time? No, but they are there when I need them and I don't have to try and remember what they were called when I need them and have to search the App Store to download them when I need them). I shoot lots of photos and video (and at 1080p60 you can be sure those videos are going to be huge). I don't use iCloud to store my photos. I like playing games and some of those apps are 500MB to 1GB or more. Even the cache files that build up on some apps like the Google app (currently at 605MB) and Twitter (859MB). Not to mention the space that can get taken up by iMessage/SMS, and email. I make a number of voice memos, I download podcasts, sometimes when I travel I will have some videos on my device etc...
I mean, I can see 32GB in 2014 being fine for most people, but 16GB (especially when it is actually less than that) would seem to be a pain in the ass to manage. Plus, if you can afford to buy a 16GB iPhone 6, you honestly can't afford another $100 to get the 64GB model? Especially considering what people pay for the cell phone service? Why not just get a new or used 16GB iPhone 5s then?
The main reason Apple even has 16GB iPhone 6/6 Plus models is to upsell people on the 64GB model. Most people would think, heck why pay $199 for a 16GB when I can get 64GB for only $100 more. That's why Apple did that.
Anyway, to each their own. If you have used iPhones in the past then you know how much space you typically use. But for new smartphone buyers, I would really think about how you plan on using the phone. If you like taking lots of photos, shooting video, like playing games and using a number of different apps, do a lot of messaging and email, those things take up a lot of space and you will find yourself having to delete things and having to pick and choose what you want on the phone which will get annoying really quick.