Cos at least the fuckers have thought about it and made a decision, rather than pretending that their decision (not to make a decision) is anything other than a useless waste.
It is a close call between the two choices, in reality. Whilst at a base level, the EU should obviously be rejected - as an enforcer of neoliberalism, and old fashioned plain free trade before '92, as an undemocratic monolith that cannot be reformed, as an emerging imperialist bloc, as an entity that works to ensure the continued impoverishment and underdevelopment of the third world - there's all that on the one side. And then, there's the fact that even if we vote to reject all that, we will still wake up in a state which is only different in that it is a fading imperialist bloc, rather than an emerging one. And the EU will offer some short-term protections against the worst of the tories, and at least raises the notion of internationalism and free movement of labour.
Those of a dithering bent (whether liberal or ultra left variety), might then try and say that' he, that means you should abstain then' - but they ignore the fact that, formally, the vote is not about endorsing the British state, or not, but about remaining in the EU. And as socialist we dont want to remain in 'the EU', so fuck that. vote out. Whichever way the result goes, it will go one way or the other, there can't be a hung referendum result, so it will have an effect. Maybe a remainer beleives it will have only a minimal effect, either now or in the long term, but that is still an effect, and we should vote in whichever way helps develop working class struggle, even if only a tiny bit. And, while you mkight say these same arguments could be made for voting in a GE (and to some extent they CAN), there is a fundamental difference in that there you are voting to endorse a specific party, not just to reject the other(s)