Chances are these billionaires are pledging to make these donations specifically to avoid taxes. I agree that the government would probably waste the tax money anyway, but ultimately the government needs some way to pay off its debt, and taxing the rich makes more sense than expecting the middle class to foot the bill.
Of course if these billionaires put significant amounts of money toward various respectable aid programs it may reduce the amount that the government needs to spend, allowing it to direct more money toward paying off the debt.
This is a common misconception about donations. Donating money never makes you better off financially because donations are eligible for tax deductions, not credits.
Choosing where it goes is a cool benefit. It's like you can pay a smaller percent and they will choose, or you can pay a bigger percent but you get to choose where it goes? It's very possible that I'm totally misunderstanding the rules.
Of course if these billionaires put significant amounts of money toward various respectable aid programs it may reduce the amount that the government needs to spend, allowing it to direct more money toward paying off the debt.