And how does the beginner know which spells are useful in the long run? The sorcerer is a one-way road. Really bad if you are still learning...
And how does the beginner know which spells he shall memorize how often before resting ? As sorcerer you have more spells and can cast them when you need them, no need to metagame and foresee what you'll need how often the next day. Remember how you played the game, your party party mage probably memorized only about half as many different spells per level than a sorcerer could pick.
And since you don't need scrolls for scribing first you have at least one extra casting for every spell available, independent of your choice.
IMHO, a mage is just a spellcasting ranged attacker in most computer RPG's.
In BG1, I've only picked or memorized Shield, Mirror Image and Dispel Magic as defensive spells, and all the rest were attacking spells, with the preference going to area effect spells (starting with Sleep, Web, Stinking Cloud, Fireball / Skull Trap, etc.). I don't actually waste one of the sorcerer's slots on things such as Identify, because I've got so much money that I can identify at will at stores.
Many non-attacking spells in BG1 and 2 are actually quite useless. Has anyone EVER cast "Detect Invisibility", or "Know Alighnment", or "Clairvoyance"? I haven't....