I actually think it's a great reward. Even if you have all the Pokemon, you still may want more--for example, if I had a Mienfoo but then came across a shiny, I'd still want to catch the shiny! Also, if you're even a semi-competitive battler, you'd be breeding a lot of Pokemon to try to get a good nature. Combine this with the Oval Charm and the Masuda Method and you might have a shiny that's competition-worthy! I also know many "shiny hunters" who stalk the tall grass of certain routes looking for a specific shiny Pokemon because they love their alternate color so much (I know I'm guilty of soft-resetting on legendaries for their shinies, too).
For me, the game doesn't instantly end after completing the PokeDex. Especially since in B2W2 there's a lot of post-game content, I'd still have lots to engage in after my PokeDex is completed, so I'd definitely be glad to have the Shiny Charm.
In SoulSilver I completed my national PokeDex just for the sake of completing it, but it was still lame that in the end all I got was a diploma that I didn't even know how to view again. Maybe to some people it isn't enough of an incentive, but to someone who already has the drive to complete the PokeDex, which is a big reward in and of itself, the Shiny Charm just makes it better! I wouldn't say it's the absolute best of rewards but to me it's still a great one.