Soaking in buttermilk is not the same as brining. To be a brine it needs to be water and salt at the minimum. Well actually you don't even need the water, and liquid would do, but salt is essential. The salt water (or liquid) is soaked into the meat thus making it juicier and more flavorful.
I agree with Gretchen that brining can make meat mushy, but only if you brine for too long. I find 2 hours plenty for my boneless skinless chicken breasts. I am a huge fan of brining. I think it greatly improves the taste, texture, and juiciness of certain meats.
Do a search here on Brining and you will find a ton more info.