... defending Constitutional institutions such as the presidential system from their detractors (here and here).
It's very easy to think the division of powers is just an archaic hassle in times like this, but it's worth remembering that if we had a prime minister instead, for example, it would not be Prime Minister Obama making sure that none of this silliness goes on. It would be Prime Minister Boehner, constantly looking to the maintenance of his coalition.
You think he's desperate to keep the speakership?!? What do you think will happen if you make him head of government.
When an institution works this well for over two hundred years I'm personally wary of dispensing with it so completely.