For me it's common sense, multinational shareholder based companies are by there very nature psychopathic, with the single aim of making more money for shareholders, if they decide they can make more money by moving into Europe if we leave the EU they will, and won't bat an eye lid and laying off tens/hundreds of thousands doing it. Multiply that by the thousands of shareholder based companies in the UK, even if it's only 25% of those big companies that decide to leave its will be carnage.
That's what I see happening if we leave. In the long term,we may prosper, but the short term your looking at mass unemployment and possibly another massive global recession.
It's not a left or right wing view, just the reality of the world we live in.