I tried to do without it during my last trip. Tried wearing pants with lots of pockets. To an extent, it worked...my camera is one of those really small digitals, my cell phone is very small, too. But there were days I really wanted to carry more, so I wore a SMALL fannypack. I have the big camping ones, too, with water-bottle holders and everything, and unless you have small children with loads of paraphernalia, the big packs are just too bulky for comfort (I have enough bulk of my own, thank you!).
Pros of going bagless...no security lines! Crucial if going at park opening.
So much freer and more comfortable! No bulk when buckling in rides.
Nobody asks mommy to "carry this, carry that".
Cons of bagless...always putting my keycard in the same pocket as cellphone by accident ( big no-no, as you can demagnetize your card that way).
Depending on the pants I'm wearing, all the weight in the pockets can cause "droopy drawers syndrome" (you think fanny packs are a fashion nightmare???).