Each scent triggers a neuron reaction in your brain...these chemical reactions are like messages. These messages can affect anything, from a hormone release to simple running. It also affects the digestion system. The scent of bananas triggers the brain to release the hormone that controls your hunger. Thus you eat less, and lose more weight!
Call me crazy, but I would never include 'smelling bananas' as a part of my daily weight loss regimen, and something tells me most(if not all) those who have attained a fit body don't make a practice out of this.
In fact, smelling the banana would only make me want to EAT the banana which would just result in me adding more calories...lol.
If this works for you though, all the more power to you, however if you're looking to lose weight, it's better to focus on getting a healthy diet and exercise regimen down to a habit...there are no shortcuts.
I reckon it is because they mess so badly that you are not inclined to eat them.