leaving brushes in water a long time can lead to splitting of the wood of the handle, or at least the paint peeling off. (Obviously the plastic handled ones are a different story).
The bristles splay when left over paint or ink builds up in the base of the bristles down inside the ferrule. That's the good of the soap and water wash. You want to get all that stuff up out of there every single time. I almost never have to buy a new brush now that I have become obsessive about cleaning out the bristles.