You are taking Danuser’s interview out of context, it’s all I am saying.
Danuser is explaining why the Void Elves numbers grew. Danuser replied that it was because many other Elves were interested in them and went to undergo a similar process.
What this interview says is that other High/Blood Elves went to study the Void, so that is why there are many Void Elves
“They start out as a small group,” Danuser continues. “But it’s natural as other elves have found out about them (The Void Elves) — elves who are interested in new sources of magic, power, or opportunities — would see if they could undergo a similar process. (Void Elf transformation)”
He says nothing else, neither implies anything else aside from saying the Void Elves are many now because many Elves went to become Void Elves. It is speculation.
As to why they are Warriors, mages, and Priests. This is simple. All Elves like magic and are attuned to it. All Elves, even the Warriors, know magic. A Void Elf Warrior can still use the Void and be a sword fighter, a Void Elf mage can still use the Void and Arcane.
Void Elf Priests are there due to the Shadow Priests, not for Holy. Just like Lightforged Draenei Shadow Priests are there for Holy. Game mechanics.
This has nothing to do with Paladins, as Paladins, worshippers of the light would not go to study the Void, they would not be “Interested in the Void Elves as to join them.”
Danuser only said why their numbers grew, not that they could use the light.