I have always found nets not all that reliable for small things. Which is where the tank bag comes in handy.
I agree, but usually I just carry a small bag that doesn't fit through the openings in the net.
But then why not have a tank bag? 
because something like a small gym bag folds flat and stores under the seat when not in use and then I don't have to take off a tank bag to refuel, or ride around with an empty tank bag scratching up my 40+ year old paint.
Haven't you used your bike as actual transportation? It's amazing what you can come up with when you need to carry something you weren't anticipating and have to improvise and then it just becomes habit. I can carry more than a tank bag, I have versatility of location, and it's way easier to remove and install, plus I carry it with me because unlike most tank bags it has a shoulder strap.
Most of the time when I was riding everyday (pre kids) I didn't need to carry anything other than whats on me, or a briefcase or messenger bag that wouldn't have fit in a tank bag anyway. I keep a tool kit, a tire gauge, and a flashlight under the seat in the channel next to the battery, how am I better off keeping it in a tank bag? If I leave the tank bag at home and suddenly I have to carry something, what good does it do me?
I have a 20 year old nylon nike gym bag with plastic fittings (its the thing on the ground full of groceries in the pic I posted earlier). Nothing to arc on to, and it folds flat and can be tucked under the seat. I keep the cargo nets on the rear grab rail and I hook one to the helmet hook so it doesn't grow legs when the bike is parked and the seat locked. That way, if the wife says, pick up a half gallon of milk and some eggs on the way home I can and not worry about the cargo net letting it fly on the highway.
The only thing I have ever lost out a cargo net was a vespa 150cc cylinder, because it wasn't in the gym bag and I hit a hard bump on the BQE. it was like a comet showering sparks as it tumbled at 60mph down the highway to the shoulder.
