So, thinking about a headcanon I have:
Halflings can become illusionists because some of them have the bloodline of the long-dead gnomes. They’re not gnomes, mind, but they have enough of a gnomish bloodline to enable them to become illusionists.
That said, however, pre-Revised, defilers and preservers can specialize (p. 26 and 28 of the first boxed set). Did y’all ever do anything with that? Any idea why it was subsequently removed?
Because revised baked in Prism Pentad lore and when it did the reasons for Rajaat fall short if the little people can cast preserver spells.
Even with revised, I continued to let them be illusionists. The idea was that the Rhulisti could not use arcane magic. Even those that did not practice liftshaping were tied so closely to it they couldn’t cast a spell. As the Rhulisti became Rhul-thaun they began to gain the ability, but culturally had no way of learning arcane magic. The degenerate halflings of the modern world can use arcane magic but can rarely find a teacher. Their use of illusion magic is tied to some now ancient teachings that mingled illusion with druidry or psionics.
Specialists exist, but are rare simply because it’s hard to find any magic to learn. Beggars can’t be choosers.
“People don’t specialize because spells are so rare” also seems like it could go the other way. “This guy specialized because 90% of the spells he had access to were Alteration, so he understands them really well.”