They don't need to, Eldritch Knight, Arcane trickster, Paladin, & ranger all gain spellcasting. The dragonmark races all get 1-2 spells they can cast 1/long rest too... not to mention dragonmark focus items. In a lot of ways, this works better than just giving them SLA alone ( they still have some Su/SLA/Ex Abilities too though)
Take a mark of sentinel EK, he's going to have compelled duel, shield, of faith, warding bond, zone of truth, counterspell, protection from energy, death ward, & maybe guardian of faith as he levels... that's a very different feel than say... a mark of passage AT who cuold have expeditious retreat, jump, mistystep, pass without trace, blink, phantom steed, dimension door, freedom of movement, and maybe teleport circle