You have to look at the entire picture. What happens is that people look at the outside funds used to pay the tax at conversion as "magic money" and don't consider the alternative use you could put it to. The correct alternative to consider is "invest it the same way". The alternative they use, though, is essentially "blow it".

I understand that multiplication is commutative, and all things being equal, paying tax now or paying tax later after growth is exactly equal (assuming equal tax rates, of course). But that isn't the case here. The case here is that you can use outside money ($2400) to pay the tax, while the entire value can move to a tax free account. That effectively allows you to add an additional $2400 into a tax free account (that you otherwise couldn't have added) to grow tax free "forever" (well until your death plus 10 years if you so choose). The tax benefit to you is the tax on everything that $2400 would have earned over all those years. If you hadn't done the conversion, that $2400 would have sat in a taxable account (or in a tax-deferred IRA) for all that time and taxes will continuously or eventually be due on it.