Are Duplicate Brushes Really Necessary?

You've been painting for years and are likely to have an enormous stash of brushes.  You've recently decided to expand your horizons and try a new medium.  Or perhaps you are just getting started and aren't sure which medium will speak to you.  You've heard talk that you should have a different set of brushes for each medium but you are hesitant--surely that advice is from a company or individual who wants to sell more brushes, right?  Well, as it turns out, no.  Keeping a separate set of brushes reserved for use exclusively with oils, acrylics or watercolors is good advice.