Transparency mode is better, but it isn't perfect. The simple fact that you have something else producing noise in close proximity to your ears is enough to disrupt your ability to hear other things, even if they're also making it to your ears with high fidelity.

I mean, noise cancelling works by producing more noise, not less; and there will be plenty of times when what's outside with destructively interfere with what you're listening to, or vice versa.

This may be hard to do in a city, but a good test is to go outside on a quiet night, wearing only on one ear, and then turn your head back and forth to alternate ears while trying to listen to specific distant sounds. It'll really highlight just how many things are still being filtered out.

Or it'll demonstrate how much more deaf you might be in one ear :P