By band I assume you mean the strap that holds the tank to the BC, rather than the bands which are used to hold together double tanks.
I agree one tank strap with a good cam buckle can work fine. I've used that setup plenty. My husband has a single strap BC that originally had a crappy plastic buckle which seemed like it would be a problem sooner or later, or he might have actually had a problem. He replaced the junky cam strap with a good one and it's been fine. I do think if you've got a BC with a soft back like some travel BCs, rather than having some kind of rigid plastic or metal support, 2 straps can feel more solid and comfortable.
Not much downside to having 2 straps, although I have found a lower strap can make it difficult to put your gear in the tank racks on some dive boats. (The racks that look carved out of a length of large PVC pipe.)