They don't hold a lot more water when they're horizontal but just a little more makes the diff.
My old original B. nodosa mounted on a vertical piece of cork and it sat and pouted for nearly 3 years before I gave it another piece of cork to grab onto, and turned it horizontal instead. Now, it's bigger, happier and blooms twice a year at least.
Since then all my cork mounts have been more or less horizontal. Some are slanted just a bit to show off the plants better, and I must admit I generally use the inside of the cork to fasten the plants to, so that I can see the pretty crenelated bark texture from underneath. Most of my orchids hang up quite high - at least above my eye level - for most of the year.
