From the companions Wyll in my view is the strongest overall class-wise, it would be a pity to leave him behind just because he is nearly blind.
Not sure I’d agree Wyll is the strongest, but he can be given the Devil’s Sight invocation to allow him to see in the dark as well. Though this would be more useful if it enabled him to see through the Darkness spell and there at least used to be a bug preventing this. I don’t know if this has been fixed.
But yes, companions who can see in the dark are less of a faff to avoid disadvantage for in the Underdark, though I don’t think it’s as dark down there as it used to be and there are plenty of ways to create light so it’s not essential.
