Happens all too often, especially when I forget to charge my phone overnight since my phones' battery last ONLY 24 hours. Luckily she's in Japan where there is a convenience store at every corner and phone chargers cost less than a boxed lunch.
Eh, ones less than 500 yen tend to be shitty, or the ones that use AAs. But you can get quality ones (Like Sony or whatever) for a little more.
But then again, I carry a 20,100mAh battery...
I would assume you just buy them to charge your phone then toss them aside once you get home. I never see them as anything more than a quick emergency charge during time in need.
The disposable battery ones never really seemed to charge a smartphone enough when I had to use them... I suppose they're more orientated towards galakei.