A somewhat different take from TPM Reader BF:
Although I get your point that the reaction among some to just walk away is a sort of emotional rather than reasoned response to political realities, just consider that this is the reaction simply among many who are in fact politically engaged. What do you think the reaction is among people who cannot afford to do anything but vote and who are, in theory, the Dems core constituency?
I remember telling a political friend after the 2004 election (we met as volunteers on the Dean campaign) that I thought the Democrats could actually win if they made a concerted effort to speak to underrepresented minorities (e.g., African Americans, Latinos, the poor) by talking about their issues seriously and making real progress on those a centerpiece of the campaign (vs. the condescending pandering with which we are all familiar). The purpose would not be to convert Republican voters but to inspire these frankly large constituencies to actually vote. Whether it is accurate or not, many of those people do not see an appreciable difference in their quality of life on the basis of who is in office. They generally get screwed either way.
I don't want to get into the possible naivete of my original argument, but the underlying point still holds. A Republican is no different from a Democrat to a lot of people who feel unrepresented either way and who don't bother voting as a result. It may look self-defeating to those of us who are more invested on a daily basis and who can discern and care about the differences between what both sides actually accomplish, however small and dispiriting they can be at times, but it makes sense to me as a way of coping with the realization that politics has failed to make a difference for you.