Sure the wraithsword is a nearly-guaranteed kill, but you're sinking a lot of points into that kill when so many units have power fists...
(Also, I know that the wraithsword doesn't give more attacks, I meant that it changes those attacks that you get.)
Magnets!!Some people model things that way because they like to take a variation of weapons.
If I model my Wraithlord with:
1x Bright Lance
(which is a totally illegal combination)
then you could set it on the table, tell your opponent (not in a tourny setting, obviously) .. "This wraithlord has the BL and EML, not the sword."
you could then turn around in another game and say "He has the Wraith sword and eML" .. you would just need to clear this up with your opponent and make sure it's OK with your circle of gaming buddies.
The area I'm in.. this would be acceptable, as long as the opponent knew ahead of time what they were dealing with.
However.. in the perfect GW world, and any GW sanctioned tournaments... that is highly illegal, and don't recommend doing it unless you want to be sent home early.
I used to model things that way so I could take them however I felt that day (or how points allowed).. but have since started playing in tournaments, and want my models to be WYSIWYG.