This is a question that should be submitted to the Sage, are spells on your Spells Known list added automatically to your Spell list?
What does this feat do for say a Warmage, where the Spells Known list is the same as the Spell list?
An interperatation of the rules in from the Thanee School of thought would lead one to say, that whatever spell a Warmage mage learned through extra spell does not appear on their spell list, and thus the Warmage knows the spell but can not cast it.
However, on Page 12 of Complete Arcane, under the spells section the Warmage has this line:
[I]" Essentially, his {the Warmage} spell list is the same as his spells known list"[/I]
Thus by the strict rules inteperatation a Warmage, ( and a Beguiler as they have this clause under the spell section as well), could use extra spell to learn any spell and be able to cast.
The poor Dread Necromancer, which I think everyone will agree is in the same caster vein as the Warmage and the Begulier does not have this line in the spell section of the class:
B]I]" Essentially, his {the Warmage} spell list is the same as his spells known list"[/I][/B]
and thus by strict rules interperatation can not use Extra Spell like it's brethren the Warmage and Beguiler.
The rules are so poorly written here that Strict Inteperatation and application of what is written yields pattently unbalanced results, and throws question as to the intent of the designers.
If learning a spell and putting it on your Spells Known list = having the spell on your Spell List, then the feat is equal for all.
Otherwise under the Strict Intpertation, this feat is Awesome for Beguilers and Warmages, that under RAW can learn any spell and cast it.