1) No language needs that many fucking ways of forming the plural. When Harry Dresden wants to change the numbers of things, he just points his staff at them.
2) Harry's bad feelings toward the accusative, after his trial at which everyone babbled Latin like it was the natural thing to do, cannot be expressed without the addition of large amounts of fire.
3) He had high hopes for the genitive. He was hugely disappointed to find out that it is probably the least sexy case.
4) The ablative takes too much explanation. If he has to choose between figuring out whether it's talking about companionship, motion away from, time during which, means by which, or any of the apparently 1000 other fucking things it can mean, Harry would rather have a beer.
5) Roman poetry is always tacking an adjective onto the beginning of a sentence and then putting the noun it goes with at the end of the sentence, five lines away. Harry prefers English, since "You are going to die a flaming death" puts the adjective where it should be.
6) Also, what the hell is this notion of ending with the verb? The verb is the action, which means it comes first.
7) Words have to agree too much for Harry's liking.
8) Latin is the parent language of Italian, which Marcone speaks, which means in some way- Harry hasn't worked out the finer details of this yet- the ancient Romans are responsible for all the ways in which Marcone messes up Harry's life and his morality system.
9) The Merlin gets to sound all sonorous and commanding in it. Chalk up another way that Latin benefits evil.
10) If faux Latin is good enough to kill (at last count) a loup-garou, a bunch of demons, a bunch of fae, a bunch of super-ghouls, the mortal forms of Outsiders, several crazed necromancers, some Denarians, and all of the Red Court, then it's good enough that Harry doesn't need to learn the rest.