I have the Ventura rack system with all three of the optional attachments, which are the Pack-Rack, the Sport-Rack, and the Grab-Handle, and a single bag that they no longer make but is similar to their Aero-Spada VII (Hyphens are in the official Ventura names.) I was lucky to buy them used from a member of this forum.
I only use the Pack-Rack even when I am not using the bag. I have only test fit the Sport-Rack and Grab-Handle. The Pack-Rack weighs very little more than even the much smaller Grab-Handle, and I use it to help move the bike around because the height of the Rack-Pack gives me a lot of leverage. Without the bag the Rack-Pack kind of makes the bike look like it has a sissy bar. I don't care, but some folks may want avoid the sissy bar look.
The longest I ever ride is 4-8 hour day rides, and for me the single bag is a good size. It gives me room for a decent sized cooler so I don't have to buy food or drinks when I add a Camelbak for water to drink while riding. And there is room for a fleece jacket, but not much more than that. I also bungee on a folding chair so that I can eat my lunch in comfort.
Having said the single bag is a good size, I would like to add a little storage for heavier items like a tire patch kit and a small pump. The Ventura double bag set up would give me all the room for that and allow me to add a rain outfit, but I would have to give up the folding chair, so I am looking at adding small panniers. That would also allow me to load the heavier items lower.
I have a set of the SW-Motech removable side racks that are incompatible with the Ventura rack mounting. I am studying how to modify them to work with each other, but in the end I may instead choose to make a custom permanent mounting for ammo boxes or Pelican 1430s. This would allow me to keep all the heavy stuff down low, and the Ventura bag would be for lighter things.