what frustrates me about the whole thing is how intelligent people can't see that there may be some merit in us leaving the EU, even if they are in favour of Remain overall.

Every single remain politician wanted to stay in a "reformed" EU. I've no idea what further reforms they might have liked (probably all slightly different) but Cameron tried and got little and clearly they all wanted more.
The best chance of reforms are now we have left - check out some of the European stuff coming out from the likes of Sarkozy. They don't want the Dutch, Danish and Swedish getting itchy feet and he's calling for a Shengen 2 zone, removal of policy making from the Commission.....