Indeed, the Casket has great advantages. The downside of course is it's low range of effect and that it's rather useless versus an opponent with no wizards or a piddly
lvl 1 scrollcaddy whose spell you can easily shut down.
Now the range factor malus can be seriously mitigated by giving the Casket to a Captain on a Pegasus (better known as a 'Casketus', which in itself is derived from 'Captasus' signifying a Captain on a Pegasus obviously).
He's extremely manouvrable meaning he can get in range quickly and often make sure he's out of
LoS of the enemy so he can't be charged/shot at. While performing his duties he also doubles as a march blocker. Firing of any spells he might steal is just an added bonus. Give him a Pistol so he has something to do during the shooting phase as well!
He is rather fragile though, or better said, the unarmoured Pegasus is. So use him carefully for if he ends up on foot he's likely to be cornered and squashed by the enemy. An enemy who will probably quickly wise up to the Casketus and in subsequent battles devote far more attention to him.
As an Empire player, might I also point out www.warhammer-empire.com ? Absolutely the best Empire specific site and back in the days when I joined (I was directed by someone from Dakka as well, just after the site was created) my generalship seriously improved, also because of the great tactics articles the site has. Have a look around, you'll see me hanging around with the same Avatar and name as well.