Income splitting can only be billed as a middle-class item. It's exactly the same as virtually any tax cut. The vast bulk of people are in the middle class, so that's the vast bulk of people it affects.
But just by definition, it affects people who have unequal distribution of incomes. As you pointed out, if both members of a couple have $30,000 of income, there's nothing in it for them. The greatest benefit is in the less equal incomes. In the extreme, if one person has a million dollars, and one has zero, they're going to have a phenomenal savings. It could affect low-income people as well. If one had $45,000 and one had $15,000, there would be a considerable savings. Obviously the split between the two incomes is where you really benefit. It's just the same as with the pension splitting that was announced on October 31.