IMO, BLs are UUs for the standard meta, and Ubers for the "UU meta" (or just a UUxUU battle, if you prefer that way). No one will care if you use an UU team with Tauros/Ursaring/Clefable/Porygon2/whatever BL to play against an OU team. Its an UU team for the standard meta. Being BL or UU doesn't matter here. Get my point?
Otherwise, if you plan to make a UU team to play against other UU team, then this is where BLs appear. You can't use them because they unbalance an UU battle, exactly how Ubers do in the standard meta. There is a great difference in making a UU team to play against an OU team and other to play against other UU team.