Yes, I think that states benefit from joining the EU due to their access to resources. The EU is a a strong coalition of countries, and any country that is lucky enough to join the EU has new access to resources such as economical resources of which can benefit that state.
Certain states to benefit from joining the EU. Eastern European countries seem to have the most to gain, allowing migration for their underutilize labor, providing a ready regulatory framework where theirs could be lacking, and allowing them to arrise from the shadow of Russia. But other countries might not be better off, having to submit to even more regulations, when they already were burdened at home, being required to admit immigrants who are not willing to integrate, etc.