Two issues I have with the 10 scale.
10 implies a difficult to attain gold standard. I look at the 10's of this gen like Super Mario Galaxy 2. Yes, an amazing game that I loved a lot, but I never felt it (or the other games I would consider a 10) were in a league above most of the 9's.
Also, a 5-scale is better for when the reviewer has a mixed opinion. Let's be honest. A 3/5 for an AAA game isn't nearly as repugnant as a 6/10. Because AAA games deserve at least a 7 or an 8. I'd rather a reviewer give a good (but heavily flawed) game a 3/5 than go with the Western grade-inflation of the 10 scale.