I did this once before but lets run it.
MoK lord w/ DW average attacks: 1/3 of the time is 0. Avg # of attacks 7.63. Abbadon is 6.66 if I remember.
So on average your only gaining one or two above a normal lord, but you have the rare possibility of gaining a lot at the cost of having none. So even though he can dish out 15 attacks on a very rare occasion the 1/3 of the time you get 0 attacks evens it out and makes the average only slightly higher than other demon weapons.
against a marine.
MoK lord w/ DW, 7.63 attacks, 5.09 hits, 2.546 dead marines.
Undivided lord w/ DW, 5.83 attacks, 3.888 hits, 2.59 dead marines(remember the +1str).
Sorcerer w/ warptime, 4 attacks, 3.555 hits, 2.666 dead marines... figuring in chance of failing a psychic test=2.518 dead marines.
MoK lord w/ Lightning claws, 5 attacks, 3.3333 hits, 2.49 dead marines.
I'm not saying don't try it out. If you can roll really good for the # of attacks it will be nasty, but if it doesn't perform well just know that on average it does about the same as the other close combat lord and sorcerer options and while it has some chance of doing great there is a great risk it does nothing :'(. I hope yours fairs better... but the above is why I really don't care for it.
After slighty changing my list I realized I wanted to change a few more things in it...After I play with this list I'm gonna try some Khorne Beserkers, and I found some extra Khornate Bezerker heads so now I'll have 10 instead of 8. Woohooo!
Zerkers are good in this codex, they should make you proud. Just don't go crazy with the plasma pistols

. Good luck in your next battle

.