Jedi, I'm not sure what you mean but Mages don't have healing spells actually...those are clerics and druids. Frankly, I disagree with you which ever class you're referring to, but I fear we'll have another Mage vs. Fighter debate on our necks (we've had about 1000 of them!)
OK in short if you're talking mages: they're indispensable for fighting other mages, quick spelldisruption, breaching through mages defenses plus thery get some decent protection spells to such as Stoneskin. I wouldn't know what to do without Magic Missiles, and Abi Dalzim's Horrid Wilting, Lower Resistance, Greater Malison, Death Spell, Chain Lightning, Limited Wish etc etc
If you're talking priests, well you mentioned some helpful spells already, namely the Cure/Heal spells. Saying 'they're useless except for healing' ius a bit silly & unfair, that's like saying a fighter is useless except for his great fighter skills...
Clerics are really kickass characters who can have decent AC and fighter skills so that they can tank for you and still cast spells. Very useful are Holy Smite, Turn Undead, Sunray, Bolt of Glory and False Dawn
Druids' Iron Skins and Insect PLague (later: Creeping Doom) absolutely ROCK!!
Every character class has some great merits.
About choosing a thief kit, if you like backstabbing, DON'T use the Swashbuckler kit, as you don't get a backstab multiplier!
Oh, and SK can also be downloaded
here
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Melusine
Your voice is ambrosia