My old bag philosophy or lack of one was always to always carry everything I might need. And I did so for way too long. It started when I went too school, I just did everything I could to bring everything I might need in order to not having to think about packing the damn thing, and always having what I needed to do whatever I had to do.
I still think that the general idea is correct.
I currently use two bags. One of them are the bag I use every single day when I go to work, and the other is when I just need to bring more than I can fit in my pockets bag. They are both picket because they have room more just what I need with a little bit margin.
What if I suddenly need to bring X? Yeah, that does not happen that often for me. And the more space you have, the more crap you accumulate to fill that space.
Here is the thing, everything I have in my primary bag is stuff I either use every day or most days. This is good for my back because the bag is lighter and it is much easier to find what I have in the bag because I can get something that fits my needs much closer.
My other bag is something I bought for when I want to carry a small camera, a notebook, a few pens and my phone. Another thing I made sure was that it is easy for bouncers to search and small enough to not fit bottles of booze or wine without being very obvious. Something that is very useful when I end up bringing it when I get drinks.
I also have a camera bag. It is useless for 90% of what I need a bag for. But it is very useful for when I want to bring my big camera, a flash and potentially my other two cameras.