I have some leftist friends who support leave, it reminds me of the "let the heavens fall" type who see only Imperalism vs. Third World and translate that into bizarrely supporting Putin and Assad and gems like that.
As much as I love to see the pigs held culpable, that doesn't mean that their enemies are preferable, nor does it mean that the whole system should be thrown out. With bodies like the UN, ICC, EU, US, their worst vagaries are usually incompatible with their principles, anyways, and eliminating these bodies gets rid of the negotiations, settlements, and the public dialogue that could lead to resolving their obvious moral failings.
Even when the outcome is bad, as it usually is with those bodies, there is a publicity to it that retains the possibility of solving the problem later.
All the BS that is happening behind closed doors with the TPP and other (worse!) proprietary deals is happening out in the open as a matter of public record and responsibility within the EU. There is no reason to go back to the 100% hidden, undemocratic model.