2K might not seem like much but certainly a help for low income families.
If you were to take away the cap and compare the numbers, there is no real opportunity to save much more than 2k until you start earning over 40k as a family. When you look at it as a percentage of income, without the cap there is an escalation of benefit. With income splitting on 20k a year you see 9% more of your income (essentially you pay no tax). At 30k/year goes down to about 7.3%, and at 40k a year you’d see 5.5% more of your income. At that point, without the cap it then accelerates. Someone earning 60k a year would see 6.5% more of their income and at 90k a year 7.2%. At the extreme end, a family earning 200k a year would see 9.3% more of their income.
So the cap is intended to skew the benefit in favour of a lower income earning family. Personally, I think the cap should be a bit higher; something in the order of 4k would see maximum benefit for families earning up to 60k a year