Bitching has a cathartic effect in general, it is a way of vicariously dealing with frustrating things outside one's control. Bitching about friends and allies who disappoint has an added value - it not only vents one's frustrations, but it also carries a potential of actually doing something, like shaming the object of the bitching or tuning other friends/allies against it.
But outside this cathartic function, bitching about Democrats is quite counterproductive. It is based on a false assumption that power resides in political parties, and political parties alone have the capacity to steer the policy direction in any way they want. Based on this assumption, the Democrats are believed to be able to steer policy in a more progressive direction, but they just won't - hence the frustration.
However, that assumption is false. It is the monied class that own most of this country and its institutions who sets the policy direction, and political parties just follow that direction, whether they like it or not. They may attenuate minor details of those policies but they are not in the position of changing its course. So blaming Democrats is like blaming soldiers for the war in Iraq - they simply follow what the powers that be want them to do. They culpability is minimal, if any at all.
I do not think that it is fair to say that Democrats suck, they are simply not in the position to do things that some people think they ought to be doing. No political party is. The best we can hope for is good Samaritanism, giving some comfort to those in the iron grip of the monied class, and Democrats perform that role well when they can. But it is unreasonable to expect them to smash the power and iron grip of that class, let alone to blame them for not doing it, because nobody can do that at the moment.
Wojtek
Wojtek