The arguments were (mostly) settled when the official WotC FAQ chimed in:
Can the warmage (Complete Arcane) benefit from the Extra Spell feat?
No. Extra Spell lets you add one spell to your list of spells known, but the spell must be taken from your class spell list. Since the warmage already knows all the spells on his class spell list, this feat has no effect.
But not everybody treats the FAQ as gospel, and rightly so (often, they provide insane and self-contradictory interpretations of the rules).
Prima facie, the line in the feat about "Extra Spell is generally used to learn a specific spell that the character lacks access to and would be unable to research" seems reasonably clear-cut: it allows you to mine other class lists, because if a spell is on another class's list and not yours, you lack access to it. If so, then the FAQ is flatly contradicting the text of the feat, so the FAQ is wrong.
However, I can see how it could have been intended to mean "lacks physical access to a written version to copy into his spellbook". It's ambiguous, but I can see the possibility.
That said, though casters don't need nice things, I disagree with the "official" FAQ answer.
There are precedents in the Expanded Knowledge, Shape Soulmeld, and Martial Study/Stance feats, which allow you to access things you would otherwise be unable to access.
There are hardly any circumstances under which Extra Spell would be useful if you adhere to the FAQ's answer. If you're on a class with a desperately limited number of spells known, maybe. Or, as the text of the feat says, if you want a spell but can't find a scroll of it. Or if you're a Chameleon and use your free floating feat every day to temporarily learn a new spell long enough to copy it into your spellbook. So the official interpretation makes it a waste of a precious feat.
Worst of all, the official interpretation is boring.
So I'm inclined, in my games, to let Extra Spell take a spell off any list at all.
With the exception of known game-breaker spells (though most of those are level 9, and thus unlearnable with Extra Spell).
And with the caveat that you don't get to pick from the weird lists like Trapsmith or Adept to get a spell early; if it's available to a full caster player base class, you get it as a spell of that level, no lower.