Surely it’s better to carry a complete tool kit, 4 changes of clothes, a replacement bike and a 4 course meal in a massive back pack than use a bottle & a pocket?
It’s not a bad idea to pack for worst case scenarios, though obviously it can go too far.
You may be warm enough in your skimpy top and lycra G-string while riding, but have a crash and it may be a different story, sitting there getting hypothermic on top of a Welsh hillside.
Depends where you are riding and how far from help I guess.
I always wear a backpack now, it’s an Evoc one with a back protector so I like to wear it even if it’s got barely anything in it.
I have an irrational disdain for bottles….and mudguards 😉