I see people with 3-4 external bags on the outside of a small-medium pack as their organizing system but some of those external attachment bags are made with heavy canvas and molle systems. And I got to ask "why?" when you can sort your gear with lightweight stuff sacks on the inside of a larger pack.

Adding pouches to an existing pack has versatility unfortunately usually at a higher weight per volume stored. It works great for smaller add-ons that you want to be separate or get access to quickly - like a first aid kit or camera equipment. Some people like to have an external water carrier -- me I prefer a water bladder and suck tube and a secondary smaller bottle in a side mesh pocket. Each to their own.