A 5 point scale is fine. If you think that a game needed to get 4.5 stars instead of only 4 then you are putting way to much emphasis on the score number. Just read the review and see what the positives and negatives are then decide based on that if you should buy the game.
Doesn't that also mean we'll have to pay for the game again?
No. If you bought Minecraft when it was in Alpha or Beta stages of development then you already paid for the full retail version. It was basically like pre-ordering but you got to play the game while it was still in development.