Although I haven't played a age character, I think it's a bit of an exaggeration that 95% of the spells are useless.
I think the problem is that starting the game at mid-level (level 7 or so) negates the effectiveness of a number of the lower level spells. Spells which are effective in games where you started off as a lowly level 1 character are of not much use in BG2 because of the tougher enemies, and hence better saving throws.
Still there are a number of useful spells, and as Drewal said, there are many others that are useful under the right circumstances.
Here are some (off the top of my head):
Magic Missile
Haste
Cloudkill
Fireball
Lightning Bolt (but watch out in confined spaces!
)
Melf's Acid Arrow and Flame Arrow (very useful vs trolls)
Otiluke's Resilient Sphere (useful to take someone out of a fight completely).
Spell turning/deflection, etc
Flamestrike
etc, etc