For years I have toured with rear panniers alone and an aero bar. It worked really well, however:
1. I could sometimes miss a handlebar bag for my camera, money and other valuable stuff. The main purpose of that bag would be convenience of just clicking it on-off when going to a shop for doing groceries. Also I would carry some of the "heavy" stuff in the handle bar bag so I could shift my weight ratio more properly. I have really missed that.
2. Oftentimes, maybe my rear panniers struggled with the volume so I was missing a bit of space. But it worked. Having my valuable stuff in my rear panniers in a waist belt kind of bag was always annoying because I felt I had to empty a pannier each time when going to a grocery store.
I have bought an Arkel handle bar bag and it is beautiful and I have made some small tours with it, but I feel I miss the aero bar. Then I see the Revelate bags which enable me to combine both an aero bag wih a handlebar bag. But due to their strapping system and that it basically is a roll only, I will miss the convenience of rapid "in and out" combined with multiple pocket for optimum organization.
I would be happy to hear about some solutions :-) (without hijacking this thread).