I would tend to agree that any economic predictions of doom and gloom on leaving are probably exaggerated, at least in the short term, but then so is anyone who is convinced that economic paradise awaits us on leaving. The cost of being in the EU (taking the Brexit arguments to be correct) is of order £10 billion (£13 gross, £8.5 net, taking the rough average *(1)); the UK's total spending in 2015 was £742 billion*(2). The direct cost, then, is not much more than 1% of the UK's total budget. I'm not sure there's any significant change to this figure if you consider EU benefits claimants. Figures vary overall, including a net contribution of order several billion, while the benefits bill appears to be estimated at about half a billion*(3). So economically, frankly, the case for leaving is at least exaggerated. In the short term, of course, severing ties with the EU is likely to be messy and costly, but both sides presumably would find it in their interest to make things as smooth as possible.
On the other hand, what can the "in" campaign really draw on? As a rallying cry, "yeah it's probably not as good as it could be, the EU, but we wouldn't be all that much better off leaving and there are various subtle benefits of membership," is really not a strong rallying cry.
So it really does come down to philosophy, and various less tangible considerations, in terms of staying. I do believe that it is the better direction in the long run to pursue closer and closer unions between the countries of Europe, and in my field at least the benefits of membership are visible. Leaving would threaten them, and while the threat may not actually materialise I don't see why it's worth the risk for what I'd regard as a step in the wrong direction anyway. But no, that isn't very passionate an argument at all. It's pragmatism.
1
https://fullfact.org/economy/our-eu-membership-fee-55-million/
2
https://www.gov.uk/government/publications/summer-budget-2015
3
http://www.express.co.uk/news/politics/618332/EU-migrants-benefits-Cameron