> Whip out your phone.

Well that scenario is more like this:

Fish your phone out of your pocket or bag, unlock the screen, pull down the notification bar, navigate to the app...

vs

Look at your wrist.

Pebble is quite convenient especially when you're on the go. Walking while staring at your phone is quite hazardous. Also, when my phone is in my pocket, I usually don't hear it ring but I definitely feel when something strapped to my wrist vibrates.