You know what a bath is like when you first get in -- lovely and warm, relaxing and refreshing. But after a while the water gets cold and your skin gets all wrinkly, so you get out. So now why do you think your brush likes to be left to soak for extended periods? Even if it isn't nasty water or turps/oil as in the photo, it's not a great idea.
For starters, the brush is resting on its bristles, which are the most valuable and delicate part. Then the water is soaking into the wood of the handle and the glue that holds the bristles in... While it takes extended abuse for the ferrule to loosen and the glue to dissolve, why risk it with a decent brush?


