I don't have to. My phone usually lasts 2-3 days on a full charge. But it fits into a pocket in my laptop bag quite nicely, so I always take it with me so I can top up my phone at work if I want to.
Nope. My 6+ lasts me all day. For example, today I got to work at around 2:00. Didn't get home until midnight. Phone was on 75% battery when I plugged it back into the charger by my bed.
I always bring it regardless but since I got the iPhone 6 I haven't had to use it. With my 5S the battery used to be near dying by around 6pm (from fully charged at 7am).