I think the real feature is that you're in full control: the Transparency option is basically the opposite of noise-cancelling, giving you full operational awareness in a noisy environment without taking the headphones out. It's even trivial to switch between them (pinch and hold the stem).
The real use for noise-cancelling isn't city-walking, but airplanes, crowded coffee shops, etc. :)
Transparency mode is great. That's what I normally use, and then only switch to quiet mode when I'm in a particularly noisy environment such as on the subway.
The real use for noise-cancelling isn't city-walking, but airplanes, crowded coffee shops, etc. :)