A Warrior hero from the Dungeon faction? That's absurd...

Letos doesn't really need much magic. In general, Letos is also a quite capable warlock-warrior, certainly no worse than Vaishan.

Does this mean that Letos won't cast anything other than lower-level dark magic?

If that's the case, then why does he get so many magic points when he levels up? After all, you don't need a high magic skill to cast lower-level dark magic.