On Magic from my experiences based on the system TDE4 (or "Why mages are not demigods in RPs") just as example:
So you are an Magican. Nice! You can throw an Fireball. Even Better!
The fireball (or any Hexa-elemental ball) blows an big hole! Total win!
So far its like any Generic DnD mage.
The Fireball need 1 1/2 Minutes preparation and there is no way to pre-stock one? Bad! The Fireball has the third of the speed of an Arrow and still needs your concentration. Bader! The fireball goes of then it touches something big enough or you loose concentration (with chance to fire back)? Worse! It uses 4/5 of your magical power to be deadly enough. Worst.
I dont know how exactly magic will work but Toady had some statements over the years on this theme maybe someone can collect and put them on the Wiki so we can lift the fog an bit.
How i (want to) see mages in an RPG Group and maybe in DF:
As you see the mage itself is an Brute of mind if it comes to battle magic yes but they are "one-hit-wonders" at best. Even if you play an "Battlemage" you are the weakest part of an fighter party cause your Asp (= Mana) regenerates only at best with an minor part restored per sleepphase. Comeing to think of it any fighter that is 2 levels below you can beat the crap out out of you if he gets nearer then 21 meters cause the fighter has the advantage of being viligiant and has something called Stamina. The second thing are the from by most nordic warriors (and related) beloved throwing axes and any other projectile thinkable that is disturbing your concentration and at worst is deadly.
What use is an guy that can trow Fireballs but only one per 5 days, if you have another guy that can kill 5 enemys per day with an sword? An average archer has many times an much higher range then any destruction spell and by 10-30 Arrows Per second an much higher fire and cripling/killing rate. Normal fighters can take more damage then the avergae Mage too.
An good startegie and 10 warriors is much more valueable then 5 Mages and no startegie. If i would hve to bet my money goes on the warriors.
So what use have the most mages? They are an Toolbox, an way to help you warriors archers and what not. The mage is mostly the groups healer if you dont have an consecrated or priest with the ability for healing. Apart from that he helps your goons with strength, Nightvison, an elemental wall to save the left flank against zombies for an short time, or an slight wind that disturbs Arrows from the enemy. Mostly the ASP of the mage halfway is gone after such actions.
Outside of fights they are even more useable the it comes to potions artifacts enchantments and many other things.
Potions: Potions are great but have disadvantages. They are like shorttime steroids. Any Asp/manaless Goon can mix one if he knows how but an mage due to his astral powers can make "better" ones. In an fight serving 3 potions of strenght is better then paying precious energy for an spell.
The disadvantages of such behaviour is that like any drugs/medicaments it can Make the user addicted to it if they not have any side effects. The other things are that an potion can go bad, Vials are expensive as well as the (rare) ingredients,they can break, they take storage places and can be heavy as well as the entire alchemical labour if you are mixing the stuff on the battlefield.
Artefacts: Siegfried had Nothung, Arthur Excalibur, the Hobbits theyr Elbian coats. Artefacts are made from pure win ... as long the number and/or power of Artefacts is limited. Artefacts are useless if anyone has an magical superscharp sword and that is an general problem of many RPs.
What use has carrying an mage with you if all you need him for is imprinted on one or another scroll, trinket, weapon etc. Just let the mage stay at home an produce more of this wonderful stuff.
Artefacts should be range from fast to do one use "scrolls" up to yearlong projects with finding ingredients, star constellations, accumulating power, the preparing your place there you do create the artefact etc.
The ways to deal with artefacts are different. From the steampunkish worlds from the MTG Novels (Mirrodin cricle, Artifacts circle, Brothers war) with an Artefacts industries and magical industrialisation over to "no artefacts" worlds.
The influx of artefacts in df can be both if toady programs it with mod-able values. The artefacts should not have only advantages or if only very restricted ones like "Steelfist, the Ban-hammer of unlifing" doubles his damage against undead but only works in serene Biomes and places with the right spheres.
The disadvantages for Potions can, depending on the artefact, work for the here too. We could tell that "Spell-matrix-destabilisation" or whatever if the artefact looses power or gets side effects over the time etc. Some artefacts should have an energy drain as long they work. What kind of energy is another question.
An by possession created artefact should have more negative side-streets depending who is possessing the creator.
Enchantments/Rituals: Same things as for Artefacts with some differences.
Enchantments/Rituals should ever be at least slightly affected by the spheres of an place. An temperature lowering enchantment should for example get an penalty for being done on an place with an Fire-sphere.
Long lasting Enchantments should drain energy from somewhere. Say if you have an strong (and surely big) field of healing over your barracks and Quarters your Mages are in general pumped out or you drain it from your unicorn herd. Draining from the sphere of your place (with weakening its effects), the temple-users or right paraphernalia (Alicorn, Dragon scales Imp-bones)should also be possible.