mystral: I have to disagree with that.
The spell to summon skeletons is great (...) and skull trap is very useful, especially in BG 2 since its damage is capped at 20d6 instead of 10d6 for fireball.

More importantly, 1 extra spell per level isn't that much, especially at higher levels, and is unlikely to make a serious difference in fights, while the flexibility of having every spell available can be very useful. That's especially true since every NPC mage is either a specialist or multiclassed, and their specialty often makes them pretty useless (Xan...) except for Edwin.

kmonster: Skulltrap is capped at 9d6 just like fireball since the maximum mage level is 9 in BG, it has a short casting range and doesn't always explode immediately. The level 5 mage spell Animate Dead is also a level 3 cleric spell, there's no scroll to learn it from for mages in BG.

Having every spell available for scribing as pure wizard can be useful in theory but with less spells to cast per day you loose a lot of flexibility and the specialization bonus is very useful in reality.