I'm used to the 5 tier system because of the way the home town newspaper, sfgate.com, reviews things. There's no 3.5 stars or what ever, it's one of 5 pictures:
outstanding - the little guy leaping out of his chair
above average - the little guy sitting up and clapping
average - the little guy sitting up paying attention
below average - the little guy asleep in his chair
poor - an empty chair
There's no "above poor" where the guy is about to get out of his chair.
Adding "above above average" or what ever really isn't going to change anything. For example, when 90% of the votes for a David Mack book are Outstanding or Above Average, that's all I need to know. It doesn't matter to me if 90% of the votes are scattered across the top 2 categories or the top 3 of an expanded list of options. And what Thrawn says is 100% true, I'm really not interested in having the ratings chart thrown out just so we can start over with a larger set to choose from.