I'm not forgetting that at all.  Baseball players play hundreds or thousands of games and yet there's no agreed upon ranking of the top 100 players in the world.  For any game that isn't strictly point based, there will be some algorithm, usually secret, to determine the rank of each player.  Certainly you can argue that since the creator of the game made that algorithm, whoever is higher in it is by definition better at the game, but I find that argument flawed because the algorithm is secret and also probably only approximates what the creators consider the best playing of the game.

Even for online games in which you can imagine a perfect set of public goals such that it's inarguable what constitutes being "better" than someone else at the game, the very fact that it's online will affect the rankings.  

At the most basic level, ping time, the amount of time it takes for packets to travel across the internet from your computer to another computer introduces too much variability in online games to rank anything accurately at the top levels where competition will be close.  People with even slightly slower connections will be at enough of a disadvantage that it will affect the game.

To make matters worse, any online game will have it's share of cheaters, which will have an unfair advantage over regular players and thus dominate the leaderboard.  In Halo for example, the host of a game is chosen based on his having a high connection speed.  If the connection from the host to the other players in the game is slow, the other players will be at a huge disadvantage.  Cheaters then can reduce their upload speed after being chosen as a host, even to the point of shutting it off completely leaving the other players unable to move while the cheater runs around killing everyone.  More advanced cheaters find it trivial to intercept packets that indicate gunfire of other players and ensure that they get dropped by their firewall,
meaning that the other player sees his gun fire but it never actually has a chance at hitting the cheater.  Even more advanced cheaters know that the game checks that the map files haven't been tampered with when loading it, but if you add and change unimportant bits in the right spot you can change important bits like increasing the autoaim so that the computer aims for you all the time instead of only compensating slightly for the poor control of the joystick without the game noticing.  Those modified map files have been leaked so now you 
can just download the right map files off the web and use them.

For more detail I suggest the comments and article at <link>
or <link> where you can clearly see that people die when he fires even though his target isn't remotely on them.

So I guess what I'm saying is, if the people at the top are almost certainly cheating, and the leaderboard only shows the names of the top few people, and it only makes everyone else feel bad for being low or gives false hope to people who aren't technically savvy enough to realize that they'll never get to the top without cheating and eventually makes them feel bad for not making it after devoting lots of time to the game: then what's the point of a leaderboard?