One man's benefit is another man's cost. Being out of the EU is a (massive) "benefit" to millions of people in itself... not to you or I though. However I will concede that there are aspects of leaving the EU that at least appear attractive, even if they will, in reality, make little to no positive difference to our country (a new immigration system, ending the jurisdiction of the ECJ etc.)
But it's not only the costs that I'm concerned about - but our future ability to mitigate them. This, however, creates something of a paradox. In the circumstances where the UK will need all the leverage and power that it can muster, it makes no sense at all to agree to a deal that is designed to make us permanently less competitive than the EU. Thus, if the UK is to leave the EU, then we have no choice but to pursue policies that capitalise on our new status as non-members of the EU. The biggest single benefit (that I can see anyway) will be liberalisation of trade with the rest of the world, which would not be possible in a 'soft Brexit' arrangement.