Why bark that floats? I heard that before but I don't know why.
The reason you only use the bark or chips that float is that you want to get the longest life out of your mix as possible. The product that sinks, bark or CHC, is already starting to deteriorate and is absorbing more water. The product that floats has not, therefore a longer life in your pot.
The soggy material makes a great mulch on anything BUT orchids.