Q: If a beguiler, dread necromancer, or warmage can¹t use his advanced learning class feature to gain spells from schools other than the ones they can already cast, doesn¹t that make it useless?
A: Not at all.
Even though the classes you list have lengthy class spell lists, those lists don't even come close to including all the spells that exist in the game from those schools. The beguiler's spell list, for example, doesn't include Tasha's hideous laughter, a fairly popular 2nd-level enchantment spell from the Player's Handbook.
Another significant benefit of this class feature is that it lets the character learn spells published after the class was printed. For example, Complete Mage includes thirteen new spells that a beguiler can learn with advanced learning.