Imagine you are framing a house. To complete the framing, you are given your choice of any one of the following saws: a chainsaw; a scroll saw; a mitre saw; a table saw; a sabre saw, a wood router; and a circular saw. Which one would you choose?
What if you were building a birdhouse? Would you choose the same saw for that job?
How about the demolition of a non-load-bearing wall? A welcome sign for your new home? Or clearing the woods behind your cabin?
Now imagine you're planning a role-playing-game session. Your players prefer copious combat options and frequent dice-rolling. On your RPG shelf you see the following games: World of Darkness; DnD 4E; Amber; Original D&D; AD&D; AD&D 2nd Edition; Exalted; an OD&D retroclone; and Pathfinder. Which game would you choose?
What if your players were more interested in character development and character/npc interaction? Would you choose the same game?
How about a player preference towards exploration and the thrill of discovery? Or a game for a group of casual gamers?
Like saws, role-playing games are tools, or systems. Each are good at performing certain activities, and have certain limitations inherent in their purpose or design. Could I use a wood router to frame a house, or a chainsaw? I suppose, though neither is appropriate to the task. I would never use a circular saw to clear the woods behind my cabin. Similarly, I would not use Exalted to play a gritty, low-level, high-mortality game, an OD&D retroclone to provide myriad complicated combat options, nor D&D 4E for an npc-interaction or exploration-heavy game session.
While it's commendable that some would like to cast a wide net, and have us all drawn into single game system that emulates all play-styles, the effort is misguided. To date there has been no single RPG that simultaneously satisfied the competing demands of complexity versus elegance, grit versus grandiloquence, story versus sandbox, role versus roll, and speed versus detail.