I skate wearing a book bag probably 40-50% of the time... I commute on my board so I have work stuff, my PAPR, winter layers etc but I also like to keep a skate tool and extra set of wheels/bearings/hardware permanently tucked into the bottom of the laptop pocket. I don't have a car anymore (it's more of a pain than a help in my neighborhood) so anything I might need for the day has to fit in my pockets or on my back.
As long as the bag is well made, comfortable, and has space then I'll adapt to it. I've tried messenger bags or those weird fanny packs you wear across your chest but they're either uncomfortable or too small to practically hold my shit.
Another bonus of the traditional style backpack is you can actually use it as a hands free board carrier. Just take one of the straps and loosen it enough so there's some slack under your armpit, then twist your wheel in it so it hangs there, pinched between the strap and your back/side for stability. Any time I want to use my phone or have a smoke or something I just hang my board there, then if I want to grab it I just lift it up and wiggle and it comes right off. It's not super secure or anything but for a ghetto quick-release system it works fine.
Then if I want to "really" skate I just take it off.