[Lazy_Perfectionist]

Well, you wouldn't always want to force it.
It's not always an improvement, and you lose any special abilities like stealth or regular leadership.

Twice Born, yes. Sometimes the wight mage is a better chassis than what you started with. There are several minor factors, but I'll focus on the big three.
1). No fear of old age.
2). Undead means 100% precision during "Darkness"
3). Occasionally (I have no idea how frequently) you'll get an extra point of death magic.

1). Plenty of fear of banishment
2). Undead means you're vulnerable to no resist spells like cleansing water and wither bones.
3). Occasionally, you'll lose a limb or get a crippling wound that renders them useless.

Crones lose nothing by becoming a wight, as well as becoming less vulnerable to (poison) assassins.