I really only recently added a caster to the party, but once I did, I found him rather useful, though I do tend to use a very minimal selection of spells.
Fastness of Body, for one, I'm finding keeps the battlemage standing "tall and proud" through most battles. I kept it at level 10, thus having an effective spell modifier of 5 and found that this is sufficient for me. Anything above that just eats too much astral energy, imo.
Skeletarius at lvl 11 is also very useful. Often enough, the skelly will rush off and kill half the mobs before my party is even engaged. Since the party is not engaged in combat, Fireball (Ignif... whatever the spell is called, I keep forgetting) takes only a signle round to cast. Skelly plus two or three fireballs got me through most of Gruldur's Deep barely singed. Just don't advance the spell above lvl 11 - then it summons a skelly mage which is nowhere near as effective.
Plumbabarm is always worth it. I kep it maxed out because the AR, AT and DV reduction on enemies, and especially bosses does wonders. Instead of a long dragged out fight, all I need to do is land a few wound-inflicting blows and the bosses are down in no time.
While I still had Gwendala in the party, I maxed the Sleep of the THousand Sheep with her. It came handy when I was being dead-stubborn about sneaking past the orcs, no matter how many reloads it took.
I never used the tactics myself, but I do suppose that maxing this one out can save the collective party's butt against tougher opponents: send them off to dreamland and buy yourself some precious time to quaff potions, heal and otehrwise boost up before unsheating those weapons again.
Fireball, the all-time favorite. Keeping it at 10 (or was it 12?) - does decent damage. Not sure if advancing the apell also increases the AP cost of casting though. If yes, I'll be keeping it as it is - it does more then a decent damage. Use with caution, though - these fireballs act as fireballs should, meaning that often enough, my entire party was on the verge of killing the mage for setting them all on fire.
Fireblast (again, forgot the actual name), also at level 10 or so targets only one opponent, but does decent damage so its worth it. Same, as far as I'm concerned, applies to both Lightning spells. AP cost is low enough to unload several of them onto the mobs as a battle opener and still have enough left to summon another skelly when the first one bites the dust.
Balm of Healing, I never upped above 6 (spell modifier of 3). It's enough of a quick combat patch and usually removes a wound or two. Takes little time to cast so a nice way to prevent melee members from dying in battle.
The spell description of Clarum Purum sugessts that the modifier should be high for this spell to effefctively remove poisoning. I kept the spell on 0 and I have yet to see it fail to remove poisoning. So, definitely a spell to be in the spellbook, but not worth investing a single point into it. Works like a charm.
Of course, you can always arrange spells to suit your playstyle. This combination suits mine just fine, but I know some people like having sperate buffer/healer and offensive casters. I hardly ever buff except for the major boss fights and of course, the mage usually has an active Fastness on him, but that's about it. A fireball here and there, perhaps a lightning to spice things up, a skelly up front and Clarum and Ba;lm if/when neded. Aside from that, I gave Jost a bow and he makes quite a decent support ranged attacker once he runs out of AP.