The problem is that if you don't remove backstab immunity for undead an undead themed adventure can easily become boring and frustrating for a rogue's player. Things like flavor or immersion are utterly pointless and even worse destructive to the game as a whole when they make the game boring or frustrating.
Which is only an issue when the only fun the rogue player gets out of the game is combat related when he stabs things for a bucket of D6 damage.
That raises an interesting question, how many D6 is fun?