One can't overstate the importance of keeping some people out of power. I'm not going to get into how crucial that is because it'd take up an entire diary, and because I think everyone here agrees. It is for this reason that the 'yellow dog' (or similarly, 'lesser of two evils') argument for supporting Democrats in general elections is so compelling - at least a cowardly woozle isn't possessed by the ludicrous 'nostalgia for an age that never existed' that would have us building machine-gun nests at the border and mandating prayer-periods in public schools.
That said - we do a disservice to those we elect by not constraining the misuse of power with legitimate accountability.
Goya's Tu Que No Puedes - "You Who Cannot"
The donkeys represent the aristocracy. That they ride humans is an analogy to the 'topsy-turvy' nature of society wherein those who work most benefit least, while those who work least benefit most.
Francisco Goya's series of capricho etchings are commentary on the society of Spain, his point of view informed by his study of the philosophical underpinnings of the French Revolution. From his perch as the court painter of the Spanish monarchy, he brought attention to wealth inequality, the horrors of war, the state's inequitable use of violence, and the illusory idealization of the aristocracy. Goya doesn't appear to have believed that loyalty to 'king and country' precluded him from criticism.
I think Goya's social criticism can be shown to have done the monarchy a favor. When a government stops listening, its critics stop talking - but they generally find other means of voicing their discontent. Goya's position as an internal critic of the aristocracy may help explain the great number of copies of the inexpensively reproduced caprichos. For people who feel disillusioned, an active ombudsperson who recognizes their concerns is fantastic - see the way Nancy Pelosi's comments about Barack Obama's campaign-promise-consistency were received by administration critics.
If we chose to eschew criticism of the Democratic Party, we would not only lose the ability to credibly attack the hypocrisy of the Republican Party, but we would remove a check against our party becoming something despicable in and of itself. Yet as Strategy Praxis eloquently argued in 'Thinking Differently', undesirable legislation is unlikely to be stopped by replacing undesirable legislators.
Systems studies have shown time and time again that if you have a system that constantly results in undesirable behavior on the part of participants in the system, the most common reaction is to replace the participants. And this action rarely if ever is effective if there are strong incentives and disincentives built into the system which reward the undesirable behavior and discourage the desirable behavior. You must instead find leverage points to alter the structure of the system.
This is good news for the 'yellow dogs' among us - while changing the name on the ballot is often easier than changing the institutions the ballot fills, we can press for changes in the institutions that maintain political accountability in ways that don't hinder the electoral chances of any Democrat worth the appellation - the 'boycott Glenn Beck's sponsors' movement is a good example, and media reform in general.
Effective institutions and other instruments of accountability can make any politician more tolerable. Richard Nixon is the reprehensible person who, in his heart of hearts believed this:
Nixon: I still think we ought to take the North Vietnamese dikes out now. Will that drown people?
Kissinger: About two hundred thousand people.
Nixon: No, no, no, I'd rather use the nuclear bomb. Have you got that, Henry?
Kissinger: That, I think, would just be too much.
Nixon: The nuclear bomb, does that bother you?...I just want you to think big, Henry, for Christsakes.
(Daniel Ellsberg's memoir, pg 418)
Yet Nixon was constrained by an active and honest media, a Democratic majority in the Senate whipped by Ted Kennedy and led by Mike Mansfield, a person who'd fought a president of his own party to deescalate the Vietnam War - and as a result, the Nixon presidency features an ironic litany of liberal successes. Under Nixon, we passed the Equal Rights Amendment, Title IX, the Environmental Protection Agency, the Clean Air Act, OSHA, and school desegregation progressed so rapidly that the number of African-American students in all-black schools went from 70% to 18% in 5 years... not to mention the fact that he offered to compromise with Ted Kennedy and allow a universal health care bill that was substantially similar to the Senate's present one, except with an employer mandate instead of an individual one.
That didn't happen because Nixon wasn't 'all that bad' - that was the power of liberalized institutions.
Finally, remember that even the originator of the phrase had some standards.
From the audience a partisan Goebel heckler rose and yelled, "Didn't you say in the Louisville convention, not four weeks ago, that if the Democrats of Kentucky... nominated a yaller dog for governor, you'd vote for 'im?"
"I did," answered the speaker.
How could he now oppose Goebel, then?
Waiting for absolute quiet, the witty Hallam replied that he did in fact say he would support the Democratic nominee even if a "yaller dog."
"But, he added, "lower than that ye shall not drag me!"