I very seldom use a handlebar bar, preferring to carry loads in panniers on a rear rack where they are stable and out of sight. Even so, I've owned one for ages, and drag it out from time to time - like for this trip because I was keen to have a little extra room and it's a handy place to keep a few essentials together.
If I had any doubts about the usefulness of a handlebar bag for touring, I'm now a complete convert. The bag holds all the little items I need during the day and keeps them within easy reach on the go. Sunglasses, gloves, camera are right there and it's a handy place to keep cash and cards. Spare clothing can be stuffed in too as the day warms up. Since it clips off easily and has a shoulder strap, it comes off the bike at stops. It does affect handling slightly, so even after getting used to that slight change, it's best not to put too many heavy items in.
As part of an ultralight touring ensemble, it's hard to beat. My only criticism is that it sits a little high. Perhaps one of the lovely Gilles Berthoud handlebar bags would take me that short step towards perfection.