Yeah, that's my general philosophy. I don't like a lot of stuff on the outside of the pack. There are a few notable exceptions, however. In winter time my avalanche shovel rides on the outside of my pack. In a burial situation, seconds count, and the advantages of quick access to the shovel outweigh my desire for a smoothly contoured pack.

If one's financial constraints make it difficult/impossible to buy a new pack, then it probably makes good sense to do a "Backpack hack". But if one can afford it, I think it is better to just buy a somewhat larger pack. Get one somewhat bigger than you think you will need. Most good packs have some sort of compression staps arrangement, and if one is carrying less stuff, one just cinches the pack down a bit snugger.

I've looked at various modular pack designs over the years, with a daypack or fannypack that clips onto a larger pack. Generally I have not been overly impressed with them. In my opinion, you tend to end up with a clunky hybrid. These designs often seem to be heavier as well. All of the extra straps, buckles, and other material necessary to make a modular hybrid just add more weight to the pack itself. In my experience, if you want a small daypack to go along with your larger pack, then a better way is to pick a good design for the larger main pack. Then carry a really light weight daypack inside of it, such as an REI Flash (weighs 11 oz) or something similar. When you're not using it as a separate daypack, you can use it inside the main pack as an organizer stuff bag. The Marmot Kompressor is slightly larger, but only weighs one ounce more.

That is my way of thinking, anyway. Your experiences, situation or choices may differ, of course.