The iPad does not have the instant availability of the iPhone, which you can always grab from your pocket.
iPads are really just lighter laptops without a keyboard, and has the limits of it, such as requiring separate carrying cases.
No one carries laptops around on the subway out in the open, they have to keep them in backpacks or cases. Same with iPads.
iPhones, on the other hand, have a completely different accessibility, and are much more readily available.
That availability makes all the difference!
(Remember when MP3 players before the iPod had tons of buttons and menus?)
You do sound like someone who doesn't own an iPad. I have one and I carry it around all the time. Not every day, but quite a bit. And i never ever put it in a bag. Why would take a 1.5 pound device and carry it around in a 2 pound bag? It's in the Apple case and I carry it just as you would a book or clipboard. It's awesome. I haven't even touched my laptop in 8 months. Its a paperweight now. I do all my real work on a 27" iMac and meetings, surfing, travel and bedtime reading is all on the iPad.