"Far-left progressives will see them as more evidence that President Obama is leaning rightward on all the wrong issues."

Exactly. No primary challengers from the left means that Obama has only to justify his policies to the right wing. Look at his record on deportations, drone strikes, prosecution of whistle-blowers, extrajudicial execution of American citizens (ahem?), etc. etc. etc. if you want to see how progressive he is. Then get bummed the fuck out for a while, then look for some interesting ways to make actual change happen. Not enough of us learned our lessons after his early FISA betrayal. Obama is no friend of the left.