Perhaps this ties in with the way fantasy magic tends to not so much use stuff. Hair from the victim, being buried under the hearthstone of the victim, engraving the name on the lead tablets -- those would all personalize the spell. Reciting some magic words and waving a wand. . . about all you can do there is add the appropriate name. Ursula K. LeGuin played with it in Earthsea, what with all the names you need to know to do proper wizardry, but neither Ged nor any other wizard has any difficulty whipping up spells on a moment's notice.
It would make a nice counter-balance between the sorcerers and the swordsmen in any world that had open and effective magic if the sorcerer had to devise a spell to each situation, even if he could throw about storms and fireballs once he did.
Would also add drama to the problem of discovering who your foe is, so you can throw spells to defend and counter-attack, lying vulnerable all the time before the discovery.
But it would certainly slow down the action.