It’s not always unnecessary - I’ve seen SACD tracks that straight up include extra pieces of music, which we can’t really store properly right now. It’s not the most common though, admittedly 
I would say that we shouldn’t force someone who just wants to enter the basic data to add both layers (so, keeping the “Hybrid SACD” format and letting people just keep using that seems fine), but I think it’d make sense to allow storing the info for both layers if people are willing to put in the effort (after all, any 5.1 recordings are supposed to be different according to our guidelines).