Fiend blooded is a great way to expand your spell list and get some stat boosts. I remember thinking the last level is a trap, but I can't remember why I thought it at the time and I'm away from my books at the moment so I can't look it up.
The biggest advantage of a ghostly visage familiar (in my opinion) is the mind blank effect, which is kind of wasted on a Necropolitan. Instead I recommend an Imp. Its poison helps in the lower levels and there's the added bonus of being able to pretend to be a Ranger when it Polymorphs into a wolf.
Probably because you lose a caster level and none of the Ability boosts from the Capstone are really relevant to an undead Dread Necro.
Giving Mind Blank to a team mate is always nice and you should not underestimate the joy of gaining complete control of a strong and stupid monster with a crappy will save. The latter goes for debuffed opponents with Craft Magic Stuff-feats. And a lot of stuff will never be able to see it coming with a touch spell if it approaches through a wall )or the floor).