Last week, in fact. I replaced my Sony Xperia V's battery. The phone feels like new again. Smartphone batteries wear out.
On a side note, I had the Samsung S3 before the Sony. Died from moisture problems. From then on I vowed to stay with waterproof phones only. Samsung evidently clued up with the S5.
I think the better question to ask would be what percentage of people do that. I am really really interested in that question. I have never had spare battery in 17 year I've been using mobile phones.
When I travel I always have 2 spare batteries, so I have assured battery live for long day of heavy usage (GPS navigation etc).
With such option you never worry about battery drain and you can freely use your device up to maximum.