Admitting our prejudices...

I don't like adding pouches to bags or backpacks -- just get a bigger pack

Sometimes the idea is to have the smaller bag attached but distinct from the main bag. A separate bag that piggybacks but can be detached and function on its own. An FAK would be one example.

My personal preference is more like TeacherRO’s — I definitely build pack loads by modularizing different parts, but I prefer the modules to be internal to the pack, rather than external.

My preference is a pack with a capacity of around 4200-4500 cu. in., designed to present a rounded profile when loaded, but with the ability to accept attachments.

Overloaded packs do not fare well in heavy brush, but there are times when you must hang stuff on, at least temporarily. An example would be going on a climb - starting out carrying your rope on the outside. When returning and provisions have been consumed, everything goes inside.

There are plenty of pack designs that incorporate a smaller day pack into a large heavy hauler, usually giving you the best combination.

I have a shoulder bag that works great in my canoe, I can strap it around the rear thwart and it doesn't touch the floor and get wet. But I don't like hiking with a shoulder pack, I prefer two straps for my day pack. And of course I'm not going to bring my computer backpack into the wilderness. So I have a couple of pocket-organizers that have most of the stuff that I need to transfer between all 3 packs.

How is this about prejudice; preference would be more appropriate.

Yes, it’s always better if your pack is large enough so everything can go inside, but an ability to attach some items to the outside occasionally is nice to have.

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.