That would have made sense if Larian would have killed off inactive party members as planned. But when you can switch Wyll in and out as required from practically everywhere this doesn't really have much impact.
It has the impact that it makes Wyll feel meaningful to your progression and gives him a role within the world. It�s not about gameplay, it�s about the character development and story. You can do the Shar related quests without Shadowheart all you want, and it has little impact gameplay wise. You still fight the same enemies and have to get past the same obstacles, but having her with you makes a huge difference to the story.
