I've generally run mine with invocation and Van hels, and then either two power stones or power stone and dispel scroll. As has already been said, it frees your vampires up to carry some more juicey items. The power stone version can be really grim, as if you have the black periapt in your list, you can potentially cast 4 Van hels just from him in one turn, although you will need a 7+ on each of them, so it is likely at least one will fail. Still, the potential to do that is pretty powerful.
As to the corpse cart, i'm not convinced its all that useful as a mount. I agree with masterspark, if you get in combat, you'll likely to be killed. T3, 2W and no save will not last long. I think even normal missile fire could take him down if its not well shielded by troops. I'd rather just put him on foot in my bunker unit. That does make for a rather centralised bloock of power dice, but then it increases flexibility in where you can put your cart. I could see the use of the cart as a mount if your not running a bunker i.e. a combat/dragon lord.