Generally, what the Mystic Adept gives up at character creation isn't spells, it's attributes. Sure, in your example they're not starting with any spells, and that's probably fine, but the real tradeoff is that they are often going to spend A for Magic because mystic adept players want to maximize their adept powers.
No one else should probably be spending A for Magic. Magicians should decide how many spells they're ok with having at the beginning, and pick the lowest Magic priority that gives them that. This is probably C for many people, or D if you're frugal and have your eye on the long term. Almost all magicians are going to spend A on attributes.
Magicians built with life path and build points get the option to spend starting karma on spells, so they'll always have as many as the player wants.
Don't ask me why those build methods have those options, I think it's just because the companion author didn't like what was in the CRB so he went off and did his own thing.
