The letter grades are only better than the 100% system in this case BECAUSE they are less "accurate" (note... you cannot be accurate in scoring a game, this is not maths)
Ignoring this fact,... the letter grades are no different.
There should not be a score that can be easily equated to a number scheme because that is what has been causing "score inflation" and discourages people from reading the content of th review, and the only purpose of a grade/score should be to help pull you in to read the review.
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I feel I put my best argument against this with my posts in this thread:
http://www.vgchartz.com/forum/thread.php?id=50667
And this statement by Khuutra is pretty good too 
http://www.vgchartz.com/forum/post.php?id=1457111
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A "grade/score" at the end of a review should not be used to compare the said game to others, because that discourages people to read the review, instead relying on the score to assume the game must be amazing/awful.
A smaller scale like with 5 points or less encourages you to read the review, because you may have been on the fence about whether you wish to buy a game, and if it gets a 4 you would then check the review to see what good/bad things in the game the reviewer found and whether you can tolerate any problems.
