Felix said:
Why? But of course, before I can let you answer that

, I must say:
I disagree.
There are many necromantic spells that can assist in a wizard's endeavors to find things out, Magic Jar springs to mind, and to aid in their general survival (
False Life).
Also, I don't think that there is any particular reason that Divination and Necromancy should be diametrically opposed schools for flavor reasons. The diviner I played and mentioned previously was a Diviner by class, Necromancer by inclination, Biologist by hobby, and Taxidermist by profession. He was trying to find the source of "Life"; the study of Death, and Life's photo-negative Undeath, provided a perspective on what exactly "Life" was.
I chose Enchantment as my banned school because I felt like that school alters the perceptions of life, but not the Truth of it. Though I suppose Illusion could be banned for similar flavor reasons, my Diviner dabbled in Illusion because of Shadow's relationship with the undead.
it's not that they are diametrically opposed so much as it is that for a rog/diviner, necromancy has the fewest useful spells. this is not to say that there are
no useful spells in there; of course
ray of enfeeblement,
ray of exhaustion, and
enervation are great for any magic user, especially one who is good at ranged touch attacks like a rog/diviner. but by the same token, you have to look at what you'd be giving up by restricting another school.
evocation is right out. one of the great things about knowing what's coming is the ability to do something about it, and the evocation school has some of the most powerful offensive spells in the game.
transmutation is a must-have also. the ability to change yourself and others fits right into the role of the infiltrating rog/diviner. no question on this one, either.
abjuration is necessary for the various protection spells it offers. leaving a spell slot open so you can take 15 minutes to memorize a defensive spell after you've scryed out the enemy's encampment will win you many friends in your party, especially after you cast
protection from energy on your front-line fighter when you've seen that the big baddie has a
flame-tongue sword. sure, you can get along without abjuration, but do you really want to?
for an infiltrator, there can be nothing more important than the schools of illusion and enchantment.
invisibility,
charm person &
monster,
displacement,
hold person, and the various image spells are vital for someone trying to get into places he shouldn't be. along with
alter self from trans,
disguse self is great for "dressing up" as someone else and ferretting out some info: just think of that crappy movie Face-Off and you'll see what i mean.
conjuration is the only borderline school in my book. the
summons and
fogs are nice, but not vital. the only reason i like conjuration for a rog/diviner is that it offers easy escape options with
dimension door and
teleport. after all, what fun is it if you can't get out of the messes you're going to get yourself into?
...of course, this is all going on my assumption that a diviner with a few (3-5) levels of rogue is the ultimate expression of an adventuring diviner. that, of course, is a topic for a different debate, but i still think that for the average adventuring diviner, necromancy is the clear choice for a restricted school. sure, you can make a very useable build with restricting another school, like the character you mentioned (nice, very unique), but i don't think it's the most efficient all-around choice.