One aspect that no one has touched on is that road bikes and mountain bikes have different conventions for the sizes of parts and that you generally can't switch between them. There are obvious things, like 26" wheels vs. 700cm wheels, and subtle things like the spacing of the dropouts, the diameter of the handlebars, and the pull of the brakes. It's like the difference between American and Japanese cars. Once you get a set of tools and parts for one style of bike it's easier to stay with that style.