The EU has made many errors in the past decade. In particular, the way the European Council (consisting of the national heads of state) are in charge of most things at the moment is at best counterproductive. I'd like to see a lot of the power shifting to the Commission and the European Parliament, and I'd like to see a clearer division of powers (so that for instance it's clear what the Greek government are in charge of, rather than heaping pressure on them repeatedly to pursue those policies that other countries think would be best, rather than their own manifesto).
However, we shouldn't forget that the EU has been an astounding success in spite of its failings. No wars on its territory, the right to travel freely and to apply for a job wherever you want, and many, many more things. If the EU didn't exist, we should create it.
There is therefore not any doubt in my mind that I want the EU to exist. I just want it to be better -- more democratic, better at providing prosperity for normal people, and more open to radical ideas like Scottish and Catalan independence within the EU.
How do we obtain such a better EU? Do we make the current one collapse and hope that a new and better one instantly rises from the ashes, or do we stay on the inside and try to reform it together with like-minded people from all the member states? The answer is, of course, the latter. If the EU falls apart, the individual countries will instantly reinforce border controls, enact trade wars and in general do many things that will make it hard and laborious to recreate a European union.
And of course the bloody Tories have chosen the worst possible time to hold an In/Out referendum! Before 2008 or so, the EU was quite stable and would have been able to deal with the consequences quite easily. At the moment it appears very fragile, however: (1) The Greek drama of 2015 seriously endangered the monetary union and has made people question whether the EU has any answers to the financial crisis; (2) the current refugee crisis is close to breaking Schengen (the open borders part of the EU); and (3) the authoritarian governments of Hungary and Poland are undermining the EU's status as a club of liberal democracies (because the rules for suspending a country's voting rights assume that there'll only ever be one "bad" country at any one time).
History is a great example of chaos theory. There are stable periods when practically nothing important happens -- of course small events take place, but they don't rock the boat -- and there are chaotic periods when one small event can have massive consequences.
It feels very strongly like we're living through a chaotic time like the 1930s (which is of course why Scottish independence nearly happened -- I doubt the indyref would have been half as successful if it had taken place ten years earlier). Of course the EU might survive Brexit, but there is a real danger it'll be the straw that broke the camel's back. If the UK votes to leave, the best hope for the EU is probably that it'll be a complete disaster so that no other member state gets tempted to leave, but that won't be any fun for ordinary people here (although that might definitely lead to Scottish independence in short order).
If Brexit is just moderately successful seen from the outside (i.e., it could be a complete disaster for most of the UK so long as London is booming so much that the country-wide statistics look OK), it could easily encourage anti-EU parties in other countries. Le Pen could win the French presidential election next year and start to implement a Frexit. And Denmark has already been quite focused on following London, so a Dexit could follow soon afterwards, too. And suddenly the whole house of cards might come tumbling down.
People who are against an organisation such as the EU existing at all should of course vote to Leave. I get really annoyed, however, when I see people advocating a Leave vote in order to achieve a better EU. It's simply not going to happen. We need to protect the EU while working hard to reform it from the inside, and to do that, we need to vote to Stay.