If they don't and they get attacked they wont have a bigger army to support them. But if they do join and they get attacked they will have other parts of the EU army to back them up. But about there fishing industry might not be as popular as it used to be,but they will be able to get more money with other industries that have been set up in the EU
During the financial crisis the economy of Iceland collapsed. The European Union is already trying to solve the economic problems in Greece, Portugal, and other countries. The EU does not need another country with a bad economy. In fact, the very existence of the EU is in question because of such economic problems.
Right now the EU's just looking to pull in more partners. With debt problem in Greece, Italy, and Spain, the EU is looking for other countries to stick their bad debt upon. It's a great idea you rob rich countries to make up for the poor countries that you allowed in the first place.