I've still got a simple, light one by Caribou. They had a range of basic daypacks and bookbags in medium-weight packcloth without tons of features or goo-gahs on them. Alas, selling $19 daypacks doesn't make as much money as selling $69 daypacks and they went out of business.
Anyway, I use it mostly to be cooler, not to save weight. Especially now that I have that 2.4 ounce sea-to-summit day pack. But for a Powerbar, 500 ml of water and maybe a hat and a phone, I don't want my whole back covered and sweaty.
It's also nice in an urban environment - I tend to unknowingly whack people when I turn around with a daypack on, much to my wife's annoyance, and if I'm concerned about pickpockets, I can rotate the fanny pack to my front where I can keep an eye on it.
For all those reasons - pickpockets, storing minimal stuff, and being cooler - mostly I've worn it on vacation in Mexico.