You can get more than one creature at a time, just not the same creature(there are also cards for this), or maybe they can't be from the same school of magic, though creatures are best at being a distraction (which is hard to accomplish reliably) because they take damage far better than they dish it. Best use for them is to trick them into walking through a bandit camp for example to get everyone attacking them instead of you which is very useful, but If enemies see you first they are useless. Once you attack an enemy they will only ever attack you.

Damage spells are unfortunately worthless. Even if you boost and stack the best of them they aren't much use except for ice ray on the end boss. Far better are freezing spells like the ice wave in water magic, chains in earth magic, and ivy in earth magic. Ivy and the freezing wave are tied in my book for best spells. Most others look cooler than they are useful. Note the freeze spell will let enemies attack you one last time but is still obscenely powerful. Think freezing 20 people in a cluster and dropping a hell bomb at their feet. Instant level up with no sword.

The only other decent spells are Earth magic's protection spells that let you take more damage. Some others might be useful if you had help, but playing though the game you are on your own so.... However, do note that I have never used necro spells so I can't speak for them.

I tried playing through as a pure mage and gave up on it half way through. Robes are expensive for what they provide and the best staffs arrive early in the game so you stop realizing any potential early on. In short its hard, expensive, frustrating, and unrewarding.

I tried as a mage. I'd advise you to take my advice from the second to last post of this thread:
http://www.gog.com/en/forum/two_worlds/anyone_got_any_tips_for_playing_as_a_mage