The only time I start to get tight on space is if I carry an extra ROM loaded as a secondary in MultiROM. Then space starts to get down around 3 or 4G free. But normally, with a ton of apps and crap stored, I'm 13 or 14G... That's enough to put me in the "no worries" stage.
The phone has about 25G of space available for user content. That's a LOT.... Yes, if you pound it full of 1g games like Hearthstone, or decide to carry around a 20G music collection, or a bunch of HD movie files, I can see where you might have issues. But I would think that is a relatively uncommon use case.