I guess that all depends on your attitude. To me "10" means perfect.. and no game should ever be given that score
I disagree. If 10 meant perfect, and therefore no game gets 10, then why not just score out of 9?
The 'out of 10' marking is way too low resolution to simply write off one of the numbers.
IMO, 10 simply means Very, Very good. Almost perfect.
As for Halo2 - depends what it does. If its simply Halo + Multiplayer, then as good as that would be, it wouldn't deserve a 10. The baseline for games is constantly shifting, and sequels especially should be marked relative to their predecessors.
So a 10 for Halo would mean that it is a massive improvement on Halo in every regard. And that is a tough target.