If that one had been prevented by France not been in the EU another one worse may have occurred.
I guess the point I'm trying to make is that there is so much unknown about both staying in and leaving that to describe one aspect, in this case staying in the EU is safer, as nonsensical based on...
You said EU security didn't shine too brightly, how do you know that, how do you know it wouldn't have been worse had France not been in the EU?
Answer is you don't, nobody does therefore to say it's nonsense that being in the EU provides added security is actually in itself nonsense!
How many terrorist attacks did France have during the period it wasn't in the EU in the last two years?
I don't think you'll be able to answer as they were in the EU, therefore you can't come to any conclusions regarding being a member state of the EU and and the decreased/increased chances of...