Versatility  is enhanced when you can strap stuff to the outsdie of the pack, but danglies make it easy to get hung up on brush, rocks, etc.  The ideal is a fully loaded, smoothly contoured pack that slips effortlessly past obstacles (I can dream, can't I??) 
 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.