Short answer: Because when they post they spur action events such as calling and writing our elected officials.
For instance, something is published, and then soon after we see a diary like this one. Not only is the writing designed to fire up your blood, but there are also TOOLS with which to express your thoughts to said elected peoples.
The best part of this particular diary (and some others like it) is what's NOT in it: There is NOTHING in this diary resembling a personal attack on the President, nothing about "betrayal" or "not voting" or "primary" or anything like that. The diary just says: "Here is the situation as I see it, here's why it's bad, and this is what we do".
And frankly, a little "ZOMG" is good for all of us.
Now, everyone here knows I'm a self proclaimed "Proud Obamabot".
So why am I praising a "ZOMG" diary? For the reasons stated above the Squiggle of Doom. It's solely related to policy and governing issues. Whether accurate or not in interpretation, it's NOT personal or "anti-Obama".
And as an aside, I really respect that particular diarist for consistently trying to keep things that way in her work. And I understand frustration can sometimes lead to hyperbole. I do it myself.
But back to my main point. Most of these diaries are calls to ACTION. Think about it. We KNOW the Republicans are constantly feeding the propaganda to the nation. We KNOW they are telling their followers to call and whine and moan. They've even written memos about it.
During the health care debate, when I made my calls to the White House switchboard, the operator was SO HAPPY to hear a POSITIVE voice. Because every person calling was totally against it.
And our elected officials LISTEN as well. Especially to the voters. We NEED to constantly be on their case to "do the right thing". Constantly.
And they NEED to hear when they do RIGHT, as well. This point CANNOT be overstated.
So, we need "the Frustrati" to help galvanize us into serious action (Professionally and courteously, of course).
I personally only wish that there be more pure policy action than descending into pure anti-Obama rants.
Frustration can go two ways. It can meaningfully direct energy into fixing something, or it can destroy it.