Erik Olin Wright has some advice (pdf, via Crooked Timber) for radical thinkers, including: "Do not let the problem of achievability dictate the discussion of viability."
Tim criticizes this, asking: "isn't achievability the most important factor?"
Think back to the 1970s. Radical thinkers, like the guys at the IEA or Adam Smith Institute, proposed privatizing utilities, breaking trades union power and cutting top tax rates.
Many at the time regarded this as unachievable even though it was viable, in the sense that an economy with such features would do OK. And yet within 20 years the unachievable had become inevitable.
This shows the value of thinking about the "unachievable." In describing a possible world, you both throw the inadequacies of the existing system into relief, and you lift politicians' eyes from grubby vote-grabbing and "kowtowing before accomplished fact", by showing that they can, in fact, do much more than they think they can.
If you focus only upon "achievability" politics becomes what it was in the early 70s - a competition between managerialists over who can manage decline least badly. And that breeds an alienation from democratic politics.
This is why I advocate policies such as workers' co-ops, basic income or demand-revealing referenda. They're not "achievable" in the sense that they can happen soon. But they show that there are more options than dull "practical" politicians think.