If you've been playing elves and skaven you might find the extra movement of skeletons much better than the slower zombies.
Who will you place on the LOS then? even Elves can knockdown your Mummies with ease on the LOS, unless you stack your LOS with all your tough players, it WILL get knocked down, and if you only put skellies on it, they WILL die horribly (again).
Scenario 1.
All your players are stacked up front, and your opponent runs his Elven Catchers right by you, and you have no chance of covering them all. = You loose, and he dodges his people on the LOS away, wich means you get to hit one elf, and he scores.
Scenario 2.
You have three Zombies on the LOS, and the rest 2-4 squares back (Mummies only 2), thus negating his speed, because you can easely cover his players when they swarm through.
The Zombies will take a beating, but with AV8, Regenerate and 30k cost, they rarely die, and if they do, you can allways buy another.
The Zombies are there to take a beating, to mark the Rat-Ogre and tie him up, to force the opponent to waste ressources trying to remove a 30k tough as nails player from the field. Skellies don't cut it, they break to easely.