The Left is not alone in using language to stifle debate and disagreement. The Right does a pretty good job of it, too. Terms like RINO, Bushbot, and similar epithets are thrown around quite freely simply to enforce rigid uniformity of thought. No disagreement, however mild, is tolerated.
More often than not, you need to look under the sheep's clothing of those tossing such terms around. I've found that many, many, of those doing so are either fifth columnists or such purists that they haven't gotten off their butts and actually worked for anything or even voted for a long time. The same people tossing that around were tossing it around when Reagan was in and still slander Reagan to this day when cornered. They don't actually care about changing things, just about playing their own little game and they are not capable of ‘enforcing’ anything unless you let them.
Of course, if you buy into the “they're all the same so we're helpless” line, so be it.