It's so rare that I make a point in commenting on the person's photo when I see a "bag+charm" combo that works well!!
I remember someone with an Épi Alma that had a cool graphic charm on it. I thought that looked really sharp and modern. In general, it seems to work best when the pairing of the two items remains clean and simple (no one wants to see a busy mess!). I think you just hit the nail on the head: it's best when the accessory does not distract from the design of the bag.
As for the bandeaux, they are nice but, again, I find them superfluous in most cases. To me, simplicity is a pillar of elegance.
Despite all of that, I appreciate seeing everyone's bags and bag charms, even when they are not my cup of tea! It's fun to see what people like what makes them happy!