There are circumstances where a mass of people want change of some kind and those in power prevent reform, so the only step forward is the revolutionary overthrow of the existing order. Revolutions are rare, painful and destructive, always throw up unintended consequences, but are sometimes necessary.
Interestingly, though, in human history to date, the most humane societies that we have ever seen were those in northern Europe in the latter part of the twentieth century. None resulted from revolution; each was the product of sustained social-democratic struggle for reform. Yes, their successes resulted from favourable conditions, but as Marx taught social conditions are everything.
I know that this next point will make Geoff angry, but I will say it anyway: reliance on revolution as the only real means of advancing human-kind has all the features of a religion. In this view whatever happens in the here-and-now is somehow artificial, ephemeral and inadequate; and only through maximum upheaval, preceded by maximum strife and suffering, can the golden age be ushered in.
Marx was wrong about two things: he underestimated longevity and power of capitalism, and he underestimated the ability of capitalism to reform when under pressure. The problem today in Britain and elsewhere is that popular pressure is disorganised and disoriented.