I took a couple of pictures of the blue cake that I did, with and without leaves on the branches. Without the leaves, it's basically a stick, but with the leaves it takes on more of the look of a real branch. The addition of the leaves isn't something that jumps out at you, but it definitely makes a difference.
So let your baker suggest leaves and don't shoot them down right away! You might want to keep a monochromatic design for a more modern look, but for more natural arrangements, leaves add an extra touch of realism.
Tip: If you're making leaves for a cake to match certain flowers, it makes the leaves more realistic to put
Another tip when you're doing a cake that has a branch on the side is to take a look at a real tree branch before you do the piping. I've seen some really weird-looking cakes that looked like they had a long brown sausage squeezed out onto it with a bunch of flowers stuck on, so keep what you're doing in mind. (I'm not saying that my branch is high art, but I know it doesn't look like a sausage.)