We all know what direction the EU is headed - the words 'ever closer union' are embedded in every single treaty. I know Cameron thinks this won't apply to the UK in future, but even if the thin gruel deal does get signed I can't help but think of that time Tony Blair came back from somewhere (Rome?) saying he'd got an opt-out for the UK over the disastrous Charter or Fundamental Rights, only for the ECJ to rule some years later that the opt-out wasn't legally enforceable.
At the end of the day, any PM can decide not to sign a treaty. They have the final decision and all member states must agree unanimously for a treaty change to be enacted. One of the things that the PM got from the negotiations was that the UK could opt out of this closer integration and that gives him bargaining power to get concessions for the UK if he decides that's right.
We can still access the single market without having to be governed by Brussels. One thing I can guarantee is that even if we do vote to Leave, we'll be selling stuff to the EU and buying stuff from the EU - all tariff free.
The European commission has been very clear that this will not be the case. We cannot be part of the single market without freedom of movement, indeed the four freedoms are what defines the single market.
The EU has no incentive and all the reasons not to make any exception on this.