Well, Magic rating has other uses as well. For example, a magician with too much spare karma could create 5 different familiars with his/her 6 Magic rating. Magic rating also determines a number of other things: Area of effect for spells, the target number for magic loss, the dice used in Supremacy Duels, it's also your "Hit Points" with regard to attacking astral objects. Basically, as meat, people are fundamentally different than spirits, however, on the Astral, your Magic rating is your effective Force rating. It determines how "big" you are and how noticeable you appear to be. People with high magic ratings and no Masking metamagic appear sharper, more brilliant (or terrifying, depending on the nature of your magician) people who have suffered magic loss appear faded, dull, and less noticeable.