[Radek]

It was only an example. Of course, your mage cannot get six level 9 spells in SoA because he will never have enough high level. Perhaps, a solo mage or sorceror can - but only a solo mage or sorceror.
But there IS a subtle difference between multiclass and dualclass that has to be considered. The multiclass is considered to be a particular class in the game and it has its proficience limits. For example a multiclass fighter/mage can put only ++ into all weapons. On the other hand, a dualclass fighter/MAGE is a full sized fighter, therefore, he can put 5+ into all weapons.
Also, the multiclass character will improve his both (or all three) parts unlike the dualclass who can improve only his active class. A multiclass fighter/mage will become both skilled fighter and skilled mage. A dualclass fighter/MAGE will be a powerful mage but he will never be a better fighter than he was when he dualled to a mage.
We cannot call all this drawbacks. Simply put, a dualclass is something else than a multiclass. Each has its advantages and disadvantages. Multiclass figher/mage is a completely different character from a dualclass figher/MAGE.